Apparently M$oft decided the bounty of running XP under Windows 7 won't work on all of the processors that Windows 7 itself will work on:
The CPU support picture for Windows 7 was blessedly simple, at least until this week, when Microsoft complicated things for one corner of its upcoming OS. This new wrinkle was the discovery that Windows 7's recently announced
XP Mode would only work on processors that supported either Intel's VT or AMD's AMD-V. The vast majority of AMD's lineup, except for Sempron, has AMD-V and will work, but the Intel situation is much more complicated, and in some ways, worse.
http://arstechnica.com/microsoft/news/2009...-7s-xp-mode.ars