For those of you who missed the update a while back, Microsoft said that they were only going to be supporting/offering XP for a little while longer. While yes, you could hack XP, as many others have done/plan to do, what good would that be? Ok, so you have one of the most stable versions of windows *currently*. If you think that having 7 year old-based software is the way to do things, by all means, go for it. I salute you for your dedication to old technology. I, however, would rather take the risk of trying something new, something that will be able to update for newer hardware a lot easier and have newer features to explore. I don't claim that Vista is the ultimate champion over XP, or that it ever will be. I only claim that it has the potential for that, and for that reason I will stick with it.
"DirectX 11 will add new compute shader technology that Microsoft says will allow GPUs to be used "for more than just 3D graphics," allowing developers to utilize video cards as parallel processors."
While my first (and probably only) purpose for using DX is for gaming, there are plenty of other uses, and Microsoft is addressing that. The gaming community often (from what I've seen) assumes that DX is solely made for them. This is not to say that they aren't a major consumer of products with regard to DX, but that there are other applications for it.
As for fast upgrades, get used to it. The world we live in is a turntable of new stuff. Consider the iPhone. It started at (i think) $400. Now the new version, which just came out, is only $200, and with more features. DX11 is probably in the first case not going to be coming out tomorrow, let alone the hardware to fully use its capabilities. If you think that the innovation for newer, better products is going to slow down, you're tragically mistaken. Now, saying that, it most likely will not exceed the pace of the crowd purchasing it.
Third: DX10/11 on XP
It won't happen. As mentioned previously, XP is no longer going to be sold soon, but also there remains just the simple reason that it would be endorsing stagnation in development of something new. Old dogs can only learn so many new tricks (pardon the terrible idiom/whatever part of speech that is).
Vista (while having bugs) and the coming Windows 7 are the future, and there is no denying that. Not simply because they have a different name, or different look, but because they are/will be made for now. And after a while, they will be replaced, with undoubted better features, new DirectX, etc. Change is the only thing that keeps things moving in this world. As IndigoAK said, yeah, they probably could have put dx10 on dos. But what would be the point? 16x AA on a 320 x 240 screen?