There's some neat directx11 demos here and directx 11 should be out in Oct or November. Certain features will probably require new cards to work and those cards will be out by Christmas, albeit at a ridiculously high price.
According to one website I visited:"Support for DirectX 10 Hardware
Quite a few features—with the exception of hardware tessellation—will be supported on DX10 hardware. Of course, DX10 hardware will continue to run games and apps in DX10 mode. But unlike DX10, which only runs on DX10-compliant hardware, elements of DX11-specific features will also run on DX10 hardware.
Multithreading will work, although deferred contexts will have to be implemented at the API (software) level rather than in the hardware. The object oriented features added should also work, though how efficiently is anyone's guess. The new texture compression formats could be implemented at the driver level, though that would be slower than dedicated hardware."
I believe that Windows 7 will come with Directx 11 installed but 11 will also be available for download on Vista.
As far as the timing goes, I wonder how much effect Directx11 will have on the graphics card market and on the game developmetn market, specifically the MMO market. Lotro was the first to support 10, followed by Age of Conan I think and then DnD Online. Lotro might be the first to support 11 also. They are big on tech over at Turbine.
I'm personally very excited for Windows 7 even though I can't afford it (or much of anything) and the tesselation and shaders performance boost of directx11.