DX 11 is in Windows 7 and IIRC someone here said it's likely to become available for Vista as well.
The new ATI 5xxx cards already support DX 11 (yes, it needs new hardware) and I'm not aware of any game available now that uses DX 11, I think the Pc version of DIRT 2 will have it (might be wrong though) and that's about all that comes to mind, it has some improvements but will probably take time to become anywhere near widespread, as has been said, not all games take advantage of 10 yet and those that do, often just use a few of the many new features it seems. (Venetica and Risen have god-rays which are DX10 only afaik but that's the only DX 10 feature I recognized in both games)