The most important is that the word 'teleology' is ambiguous, and was already ambiguous in Darwin's time. By 'teleology' you can mean Paleyan teleology, in which organisms exhibit a design plan that is explained by the imposition of a designer in a special creation at some point in history. There is no question that Darwin rejects this, and rejects it vehemently. The Origin is in many ways one continual attack on this view.
But 'teleology' has another meaning, and certainly another meaning in Darwin's time. It helps if we go back to Georges Cuvier, the zoologist. Cuvier formulated an approach to zoology based on what he called conditions of existence. As he puts it in his work, The Animal Kingdom:
There is, however, a principle peculiar to Natural History, which it uses with advantage on many occasions; it is that of the conditions of existence, commonly styled final causes. As nothing can exist without the re-union of those conditions which render its existence possible, the component parts of each being must be so arranged as to render possible the whole being, not only with regard to itself but to its surrounding relations. The analysis of these conditions frequently conducts us to general laws, as certain as those that are derived from calculation or experiment.
In other words, from the existence of a thing you can infer, in a general way, the conditions that make its existence possible; in particular, the parts of an animal must act and interact in such a way that the animal can actually exist. As we study how action and interactions make the existence of particular animals possible, however, we begin to see general principles governing these conditions of existence, and this is how we understand both how animals work and how they fit into their environment.
In the nineteenth century, the methodological approach warranted by the principle of the conditions of existence was often contrasted with that warranted by another principle commonly appealed to for methodological purposes, the principle of the unity of type, most commonly associated with Geoffroy Saint-Hilaire in his work The Theory of Analogies. Biologists who worked from the principle of the conditions of existence were continually trying to understand the physiology of organisms in terms of its fitness for the organism's kind of life. Biologists who worked from the principle of the unity of type, however, set aside any such questions of fitness and focused on trying to understand the organism by comparison with other kinds of organism. To put the matter in a crude way, the followers of Cuvier tried to explain animals in terms of how they had the means to the ends of survival and reproduction; the followers of Saint-Hilaire tried to understand them in terms of analogies to other kinds of things (the sort of analogies they tended to focus on are those called homologies by biologists today, not the ones that modern biologists call analogy; the modern terms are descended from Richard Owen's idiosyncratic usage). The disputes between these two groups were often acrimonious; and thus Teleology, as the conditions-of-existence approach was sometimes called, was in opposition to Morphology, as the unity-of-type approach was sometimes called. Teleologists often accused the Morphologists of making the unphilosophical (as we would say today, unscientific) assumption of a universal plan that, if anything, needed to be proved rather than assumed, and in general seemed to require an immense amount of equivocation; Morphologists often accused the Teleologists of making the unphilosophical assumption that reverses the proper order of cause and effect on an ad hoc basis. To use an example used by Saint-Hilaire, Teleologists would explain the physiology of a fish by saying that, since it lives in water, its body will differ from organisms that live in air so as to give the fish a better means of movement in a thicker medium. Morphologists, on the other hand, would compare fish to organisms that are most like them in order to identify a type, a sort of abstract form that they both share (with variation), and thus to explain them by reference to this abstract plan. The 'plan', it is important to note, had nothing to do with divine plans; it was merely an abstract structure established by analogy, and assumed to trace out not divine action but the 'affinities' of the structural parts of organisms, i.e., the tendencies of these parts to unite in this particular way rather than that particular way. Some later Morphologists, Richard Owen especially, started drifting vaguely in the direction of understanding these types in a Platonic way, but this was not essential to the view itself, but simply an attempt to get a better grasp on how the type turned out to be so useful in explanation.
In Britain particularly there developed a sort of alliance between Teleology and Natural Theology, and advocates of one were often advocates of the other. We have to head off a potential misunderstanding right at the beginning; while Paley was the most widely read of the Natural Theologians, his was not the only view. On a Paleyan view, adaptation of organs and organisms directly involves us in an inference to a designer. But it was possible to have more nuanced views. William Whewell is a good example. Whewell argues in favor of Teleology over Morphology, and thinks that adaptation shows the existence of a divine designer, but his view is inconsistent with the strict Paleyan view that the mere existence of the former forces an inference to the latter. On Whewell's view, Teleology is at the level of 'first induction'; applying the idea of Final Cause to nature does not involve any appeal to divine design. Natural Theology, on the other hand, is at the level of 'second induction'; it arises after you have done the work of first induction ('after' here is a logical posteriority rather than a temporal one). In first induction you try to understand animals and plants, and you find the idea of Final Cause, as clarified by Cuvier, useful for doing so; in the second induction you try to understand why the idea of Final Cause is so useful for understanding animals and plants. There is a connection, but it is not so straightforward as Paley suggests. And there were many views besides those of Paley and Whewell.
People began to get tired of the dispute and looked for a way to resolve it. Enter Darwin. Darwin was a Teleologist from the beginning, in the Cuvierian sense of the term; he only gradually came to appreciate the value of Morphology. As he worked on his theory of natural selection, however, he began to recognize that it provided a way to see how Teleology and Morphology fit together. This culminates and stabilizes in the famous passage from the Origin of Species:
It is generally acknowledged that all organic beingss have been formed on two great laws--Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during past periods of time: the adaptations being aided in many cases by the increased use or disuse of parts, being affected by the direct action of the external conditions of life, and subjected in all cases to the several laws of growth and variation. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former variations and adaptations, that of Unity of Type.
In other words: (1) Both Teleology and Morphology are legitimate approaches to biology; (2) Natural selection working on variation and descent explains the effectiveness of both Teleology and Morphology; (3) The theory of natural selection shows that Morphology is in fact a subordinate part of Teleology. To get Darwin's full view of the matter we need only add (4) Morphology, especially as understood by the theory of natural selection, shows the falsity of Paleyan approaches to Teleology.
Very few people were wholly satisfied with this. Morphologists often criticized Darwin for being pure Teleology; Teleologists, especially in Britain, had become used to the alliance with Natural Theology, and were often suspicious that Darwin was really giving away the whole store to the Morphologists. But early on supporters of Darwin began to note the value of Darwin's theory as a way to integrate Teleology and Morphology into one scientific approach. Asa Gray would specifically note this in his article on Darwin:
Let us recognize Darwin's great service to Natural Science in bringing back to it Teleology; so that instead of Morphology versus Teleology, we shall have Morphology wedded to Teleology.
Gray has a much more expansive view of natural selection's relation to teleology than Darwin does, so one might try to write this off as an idiosyncrasy of Gray. But Darwin wrote a letter to Gray in response, agreeing with precisely this point. And even Huxley, who often uses 'Teleology' to mean the Paleyan interpretation of Teleology, will say, in his comments on Haeckel:
Perhaps the most remarkable service to the philosophy of Biology rendered by Mr. Darwin is the reconciliation of Teleology and Morphology, and the explanation of the facts of both, which his views offer. The teleology which supposes that the eye, such as we see it in man, or one of the higher vertebrata, was made with the precise structure it exhibits, for the purpose of enabling the animal which possesses it to see, has undoubtedly received its death-blow. Nevertheless, it is necessary to remember that there is a wider teleology which is not touched by the doctrine of Evolution, but is actually based upon the fundamental proposition of Evolution.
Thus the question of whether Darwin is a teleologist depends on exactly what you mean. If you mean that he supports teleology as interpreted by Paleyans, he is naturally not one. Indeed, between natural selection and the problem of evil, Darwin often has doubts about the whole project of Natural Theology. But Paley's interpretation was not the only one on the table. Cuvier's theory of the conditions of existence was also a teleological theory, and considered structurally on its own it required no claims about God or divine design, because it was about the more proximate ends of existence and life. It is explicitly so in Cuvier, and followers of Cuvier like Whewell took Cuvier to have done the world the favor of clarifying how final causes worked. Darwin understood himself to be following and improving upon Cuvier, and regarded the theory of natural selection as a kind of teleological theory; and he is followed in this by his early supporters. Whatever changes in evolutionary theory may have occurred since, there is a perfectly straightforward way in which Darwin's own theory of natural selection was a teleological (even if entirely anti-Paleyan) theory.
When we look back on the period we have an excessive tendency to think of the problems in terms of narrow oppositions. But in Darwin's day, all of these things -- views of teleology, views of morphology, views of natural theology -- were undergoing massive transitions, and so, while we can still make out definite camps, there was a wide diversity of views on every single important point relevant to these. Simple binary oppositions fail in such circumstances. Was Darwin an anti-teleologist? Yes, given some understandings of teleology, and explicitly so. Was Darwin a teleologist? Yes, given some understandings of teleology, and explicitly so. It is we who think of such things in terms of a clear-cut and simplistic 'for' and 'against'; this is anachronistic when we are speaking of the real nineteenth-century. As Lewis Ezra Hicks noted even then, writing in the 1880s,
When Dr. Hodge says that Darwin rejected all teleology, his notion of what teleology is differs toto coelo from the notion of teleology entertained by Dr. Gray: still more does it differ from the " wider teleology" of Professor Huxley. So that, strictly speaking, these witnesses all agree in their testimony. Gray and Huxley would accord perfectly with Hodge in the opinion that Darwin rejected that form of teleology which Hodge regarded as the only possible form of it. But these evolutionists maintain that there is a wider and better and more far-reaching teleology, which, so far from being the object of Darwin's hostility, was in a sense created by him.