It's from a Microsoft Flight Simulator related blog, so games focused, but there is some useful info on the Microsoft Windows Vista DirectX Graphics system changes.
“Vista will include the next generation of DirectX--Direct 10 (a.k.a. DX10). The current version is DX9 and it's available on Windows XP. Vista will support both DX9 and DX10.
Why both? The biggest reason is that DX10 will not work on currently available graphic cards. It's just changed too much to be compatible”