According to Daniel Moth, Principal Program Manager Lead at Microsoft, the next version of DirectX, 11.1, will be exclusive to the new Windows 8 platform.
On the Microsoft Answers forum, Moth writes:
DirectX 11.1 is part of Windows 8, just like DirectX 11 was part of Windows 7. DirectX 11 was made available for Vista (bing it), but at this point there is no plan for DirectX 11.1 to be made available on Windows 7.
DirectX 11.1 will feature improved rendering and prettiness, native support for stereoscopic 3D, plus a bunch of other things that are slightly less interesting to most gamers. If you are not most gamers, you can check out the full feature list on Microsoft’s website.
The lack of backwards compatibility is a shame for Windows developers, who will now have to support even more versions of DirectX if they want to be compatible with older Windows versions. Dang.
Also, the phrase “Bing it” should forever be denied a place in the English phrasebook. Microsoft, stop it. Please. You’re hurting me.