This
Gamasutra article, also from Microsoft's Gamefest 2008 developer conference
in Seattle has word that DirectX 11 has been announced, though no specific
release date is specified. Unsurprisingly, this will be for Windows Vista and
"future versions of Windows," but not Windows XP. The good news is that there
will be no new hardware requirements, so DirectX 10 hardware should be DirectX
11 compatible.