We have learned that you won't have to wait for Windows 7, codenamed Vienna, to get to DirectX 11.
Most games that currently support DirectX 10 are backward compatible with DirectX 9 and there is a lot of resistance toward migration to Vista simply because of Aero and DirectX 10.
DirectX 11 will support both Vista and Windows 7 and therefore can and will be launched before new Windows. This happens simply because graphics guys and the gaming industry want to get things to the next level and some elements of DirectX 11 will get things such as tessellation to the next level.
As you're probably guessing, DirectX 11 graphics hardware comes around the same time, late Q3 or Q4 2009.
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Graphics
DirectX 11 comes in late 2009
Won't wait for Windows 7