Even though rumors were pretty confirmed, DirectX 10 will be one of the Vista-only technologies. Previous rumors were saying that DirectX 10 would be available to Windows Vista, and its Windows XP version would be scrapped.
Microsoft made its official announcement a few hours ago, and nothing new has been announced apart from the scrap of the XP version.
According to Microsoft, "The current driver model on the Windows XP is the limitation. One of the key points of Direct X 10 is to be the ability to have more independent objects and the current driver model on Windows XP simply won't be able to deal with it. "