Alan Rhoda has an interesting discussion of 'pure act' in Thomas Aquinas (HT: FQI). The issue is complicated by the fact that Aquinas recognizes a category called 'active potency', which is often what is really at stake when we talk about 'potentiality'. It contrasts, however, with 'passive potency', which is potentiality in the proper sense. Thus we cannot assume that merely because we characterize something as a 'reception' or 'undergoing' or even 'potentiality' that it would contradict the pure act thesis. This, in fact, becomes especially clear with personal activities: welcoming involves reception, but it is an active reception: in other words, it is a type of actuality. Its actuality is properly and directly signified by saying that it is, or it exists, or it has being, rather than by saying that it (merely) could be, or is (merely) able to be, or (only) can exist.
The relation between the term 'perfection' and the term 'actuality' is easier, because they turn out to be the same thing given the Aristotelian account of motion. 'Motion' in the Aristotelian sense involves a point from which the change begins and a point at which it is completed; this completion is just called perfection, which is why (in certain contexts) 'perfectio' can often be translated as 'completion'. If something is perfected, it has become actually X (or X 'in act' as it's often translated), where X is the end of some some change from potentially being X. In the case of God, the term 'perfection' can't have precisely this meaning; this is, in a sense, the whole point of saying that God is 'pure act': God can't be perfected by anything. This is why Aquinas has to take the trouble to consider whether God can legitimately be called perfect.
On the issue of determinism, Aquinas too holds that we are self-moved movers; he simply denies that we are first movers. In other words, our self-motion is not a violation of the principle that everything moved must be moved by another; it is simply an instance in which the thing being moved actively contributes (insofar as it is already actual) to the particular form its being moved takes. So I don't think there are any deterministic implications lurking here. But this is a complicated issue that was hotly debated in the scholastic era; Scotus, for example, insists that self-motion does violate the principle that everything moved must be moved by another, and so proves it to be false. In either case, however, God is not considered to be self-moved; he is first mover. When he creates or acts in relation to creation he doesn't need to actualize anything in Himself (there is nothing actually in Him to which we can refer and say "Now He merely can be this, now He actually is this"); He actualizes the potential of other things. So (to put it roughly) when God creates, He doesn't (properly speaking) actualize his potential to be Creator; He actualizes the potential of other things to be created. In other words, we can't assume that such a change involves a change in divine states rather than a change in states of non-divine things. There are a lot of complications that would have to be taken into account here. But I think that the pure act doctrines -- simplicity, infinity, immutability, impassibility, eternity, omnipresence -- are all easy enough to defend; the difficulty lies not in defending them, since there's lots to be said for each when they are properly understood, but in avoiding slips into equivocation (because, if they are true, God is very different from most things we experience, and so we must proceed very carefully in talking about Him).
There are lots of complicated issues and puzzles in the pure act thesis, both those above and many others; I'm glad people are discussing them.
