Your being a bit harsh there MS do a lot of bug fixing which strictly speaking isn't there fault/problem ie its to do with someone else's software/hardware.
The reason they don't want to make DX10 available to XP users is because they want to sell more copies of Vista plain and simple.
I can see them doing it in the future but not until they are finished pushing Vista I don't see the point in swapping OS just to get DX10 in my opinion it just isn't enough of an improvement to justify the cost.
And yes i have seen a PC running it and yes its pretty but i just don't think its worth it.
Mactronix