The recent discussion of eternity and time both here and at FQI has made it clear to me that a major issue in the debate is the question of what we should see a theory of time as doing. I would suggest the following considerations as food for thought. All of them are rough, and most of them are controversial.
(1) It's actually very difficult to distinguish the two major theories of time. Everyone agrees that time is characterizable in terms of past-present-future (tensed properties); and everyone agrees that time is characterizable in terms of earlier-later (the so-called 'tenseless properties). Thus, the usual way of distinguishing the two theories in these terms doesn't actually shed much light on the subject at all. Claims that the two forms of characterization are not intertranslatable have always turned out at some point to fail; and while this is not proof that the claim is simply false, it should make us very wary of people who attempt to do philosophy of time by way of grammar.
(2) It goes without saying that a theory of time can only tell us about temporal things. Thus, we cannot assume that someone with either theory will hold that his theory of time covers everything that exists, unless it can be assumed that everything is temporally conditioned. In some contexts -- like questions of divine eternity, or mathematical truths, for instance -- this cannot be assumed.
(3) The common idea that the Special Theory of Relativity can't be accepted in a realist way by a presentist is simply false, as can be seen in the case of process philosophers, who (universally) are presentists and (generally) accept the Special Theory of Relativity in a realist fashion, because they follow Whitehead's recognition that presentism doesn't require one to say there is only one privileged present -- in other words, you can be a presentist and hold that there are many privileged presents. The common claim assumes that to have a privileged present you can only have one present (otherwise it's not privileged); but the process view recognizes, rightly, that an A-theorist only has to view the present as privileged over the past and future. To put the matter crudely, an A-theorist can believe in a 'smeared' or even a disjointed present. An A-theorist does not require absolute simultaneity. And, strictly speaking, an A-theorist doesn't even have to privilege the present at all. Some A-theorists, e.g., growing block theorists, privilege the past as well as the present -- they simply deny privileges to the future. It is possible, as well, to have an A-theorist who doesn't do so, but grants privileges to all times. See the following paper by Dean Zimmerman on this point: The A-theory of time, the B-theory of time, and 'Taking Tense Seriously' (PDF). Most card-carrying A-theorists are presentists; but not all. This has to do with the problem of distinguishing the positions, which I discuss below.
(4) The idea, occasionally (but fortunately not commonly) found, that the B-theorist is somehow arguing that time is illusory is not even coherent. The B-theory is not an argument that there is no time; it is a theory of time, and thus presupposes that there is time.
(5) What, then, is the real difference between A-theory and B-theory? My suggestion is that there is no clear difference, has never been a clear difference, and that the distinction is just not a useful one to make because it can only be made in superficial, purely verbal, ways that have nothing to do with the price of potatoes. That's a highly controversial claim to make; but I've never seen any reason to think otherwise. The original distinction was made in terms of translation, which made sense: it was just a distinction between two views about whether 'past-present-future' (PPF) or 'earlier-later' (EL) were superior ways of talking about time. The A-theorists were ones who claimed that some facts characterizable in PPF format were not characterizable at all in EL format; and the B-theorists held the reverse. These were, it should be noted, not the only possible positions; it is also possible to hold to the position that PPF and EL are, given reasonable suppositions, perfectly intertranslatable, and the only translation difference between them is the commonplace one that for identifying some facts PPF is simpler and for identifying others EL is simpler; likewise, it is possible to hold that each is able to characterize facts that the other is not, and thus that neither was superior. But one can see how the distinction makes a certain amount of sense. If PPF is able to characterize facts EL is not, or vice versa, then those facts in contention could serve to distinguish the two theories. If people privilege PPF over EL in characterization, they are A-theorists; if the reverse, they are B-theorists.
At some point, however, probably with Mellor, it was recognized that, while the translation approach made sense, it couldn't do what it was supposed to do without begging the question. As Mellor noted, suppose we have a statement in PPF format that can't be translated into EL. What difference does it make? A statement that can't be translated into EL could still be made true by a fact that is more adequately characterized in EL format. In the same way, a materialist might hold that while first-person talk can't be adequately fleshed out by any translation into third-person talk of which we are aware, we nevertheless have good reasons to think that all first-person talk is made true by facts most adequately characterizable in a third-person way. In other words, an inability to translate everything sayable in PPF into EL could be due to a defect or limitation in the PPF format (vagueness, or simplification, or whatever). So the translation approach fails to give us an interesting distinction. However, what are we left with? Not much. For we need some non-question-begging way to identify whether a given fact that makes a statement true is more adequately characterized in PPF or EL, regardless of whether the statement (if in PPF format) is translatable into EL format, or vice versa. We have (let us face it) no way of doing this.
(6) The distinction, then, is one that deserves to die. But I don't expect many people will be rushing to kill it. Nonetheless, it is possible to work with it if we see the distinction not as a a theory of time but as a grammar of temporal existence. The difference will be this: The A-theorist holds that, for temporal things, those things are most properly said to actually exist or occur that are present. The B-theorist holds that all things at any time are equally properly said to actually exist or occur (with the understanding that they do so at the time they do). The difference is one of emphasis; the dispute is a purely pragmatic one; it has its uses, but let's not get very excited about it. It turns out to be entirely a matter of convenience of analysis.
