WARP is C++ AMP’s CPU fallback solution that uses multi-core and vector instructions. It is a really fast emulator. (Aside: REF is also an emulator that you can target from C++ AMP, but it is slow and only useful for diagnostics).

For all practical purposes you should consider WARP as a component of Windows 8, which in theory could be shipped on other platforms. However, at this point there is no plan for that to happen (this is a decision owned by the Windows team and it is out of our hands). I think this answers the core of your question. (Aside: the same statement is true for REF).

As you figured out WARP is part of DirectX 11. With DirectX 11.1, WARP was enhanced with (among other things) support for DirectCompute and hence it is one of the accelerators for C++ AMP. 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. (Aside: REF has also been enhanced with DirectX 11.1 to be a debugging target of Visual Studio 2012).

Should the plan change and either DirectX 11.1 as a whole, or WARP on its own, was made available on Windows 7, then you would have this available to target from C++ AMP. (Aside: the same statement holds true for REF). If the plan changes, you can bet we will be blogging about it (I personally am still crossing my fingers, but not as tightly as I did a few months ago).