Contary to what Product Marketing Manager for Windows, Ben Basaric had to say earlier in the week, it is now confirmed DirectX 11 (the latest API collection) will be built into Windows 7. DirectX 11 will also be available for Windows Vista users, though it's not clear whether it will ship with Vista SP2.
Phil Taylor wrote in his blog that DirectX 11 will have a better compatibility with DirectX 10 hardware than most expected. There are parts of the new API that do require new hardware support and will only run on DX11 class hardware. Taylor pointed out that the hardware tesselator and Shader Model 5.0 are examples of features that will be hardware dependent.
He also mentioned that this makes it clear that Microsoft has not "forgotten" PC gaming, which some critics seem to think.