The parties involved would never admit whether this Inquirer story is true, but it's entertaining enough that we just had to link to it. Grain of salt and all that (the article lists no sources, for example) but the theory as reported says that Microsoft started off with sound technical reasons for making DirectX 10 exclusive to Windows Vista. Due to alleged driver troubles from Nvidia, Microsoft is supposed to have loosened the requirements, and now its supposedly very possible to port DirectX 10 back to Windows XP.
Even if DX 10 on XP can be done, Microsoft won't allow it for business reasons--it wants people to buy Vista, or so the story goes
One of our missions out here at E3 this week is to talk to as many PC gaming developers as possible to find out what the near-term future of DirectX 10 looks like, especially for the purposes of benchmarking PCs and graphics cards. We don't expect to hear from anyone about DirectX 10 coming to Windows XP, but we do expect plenty of "no comments" when we ask about this story.
Source:→ C|netMicrosoft, Windows XP, DirectX 10, DX10