Vista is built on DirectX 10 and the larger Windows Graphics Foundation (WGF). While intended to be totally backward compatible with DirectX 9, apparently DX 10 is not. I do not find that surprising, really, considering the complexity of any display system and one intended to do so much is probably just that much more liable to contain bugs and conflicts.
All of this reflects on the graphics chipset maker's efforts to deliver good drivers. It's a very difficult and complex chore at the best of times and made even more difficult (and perhaps temporarily impossible) by having to work around bugs in the OS and display system code.
Contrary to the opinions of some (loudly voiced) "authorities" there is no evidence it cannot all be made to work. Obviously it is going to take time, though. How long I do not know and am not willing to speculate. A month or two should see very substantial progress. Beyond that, I have no notion.
Microsoft claims to have done extensive research, testing well over 1000 games. They say most work on Vista. It would be nice if they would share their results with us, but I expect they refrain from doing that so as not to embarrass the game companies. The same goes for their silence on which applications from other companies are truly Vista compatible or ready.
This is the time to lean back, work on our XP machines and thank the powers that there are early adopters who go through all these travails ahead of us. It's not time to upgrade yet, obviously.