Adoption of Vista is going to be slow (apart from people who buy a new PC and will have it forced on them) - Vista has lots of cute eye-candy but there's no compelling reason to upgrade, it isn't like going from 98SE to XP. The Home edition is so crippled that it won't run Aero Glass and upgrades to Pro and higher are very expensive. Established users have little real reason to change this time around and MS knows it.
Apart from Direct-X 10, Vista will have Direct-X 9cL for cards that are not Direct-X 10 compatible - DX9cL will allow them to access most of the capabilities of DX10 without forcing people to buy a new card.
Game writers will produce releases that work with DX9cL - no-one is going to release DX10 exclusive games for quite a while, simply because they just won't sell in numbers (and miss out the DX9c market which'll be around for quite a while yet).
So go to Vista if you must, but don't put off buying a high-end card by the DX10 story - it really won't be a big factor for several years. Chances are, any good card you buy now will serve you well until it dies.
PS: M$ won't backport DX9cL to XP - they say it "too difficult"
- the truth is that they could but won't, in order to drive more sheep to Vista and fill their bloated coffers still further.