That is true, Mach does provide a certain level of abstraction for what is running on top of it. However, I think the problem lies in the fact that much of the code that has been 'ported' from OS 9 onto OS X as Carbon is not so easily abstracted. If memory serves me correctly, much of that...
Sorry, it ain't gonna happen. Not anytime soon, anyways. Apple would be crazy to be spending resources on porting new technologies to Intel. If anything, they have been keeping the new code as cross-platform as possible all along.
And you have to remeber that Apple doesn't really want to...