With Apple keeping a tight reign on Mac expansion capabilities, don't ever expect to be able to get Mac versions of every GPU that comes on the market. The best we can hope for is 1 or 2 additional options and faster updates (and even that is a stretch).
All I expect is that a card that should work in my Mac will work. It seems that previously the cards physically were not capable of working in older Macs. This isn't the case now. Apple doesn't need a version of every card on the market. But so long as Apple is releasing cards that are physically compatible with PCI Express 1.0, there's no reason they shouldn't work in any Mac Pro. When Apple releases a card and there are actual technical reasons that the card won't work, I won't complain. The pre-2008 and early 2008 Mac Pros just aren't different enough that there's any good reason for cards to work in one and not the other.