There are still many gamers who have yet to upgrade to Windows Vista. Part of Windows Vista that was most appealing to gamers was DirectX 10 compatibility. DirectX 10 is something you can’t get on Windows XP.
According to AMD when Windows 7 hits market, widely believed to be mid-2009, DirectX 10.1 will be fully supported. According to AMD DirectX 10.1 will be used in the API for the Windows Desktop Manager in Windows 7.