I think I just answered this a couple posts below. Officially, right now, Apple is not preventing anyone from running Window's on the new Mac's. However, Apple and Intel are using newer technology (EFI) that the current Window's versions do not support. So, unless you want to wait for an emulator like Virtual PC to be updated, I doubt you'll be running XP on these new Intel Mac's.
At the end of this year (or whenever Microsoft finally releases it) the new Window's OS called Vista (longhorn) will support the newer technologies and "might" run on the Intel Mac's. How well it'll run is a big question mark (if it runs at all), but Apple says the won't prevent it (nor help it). On the flip side, don't expect to see OS X on anything but Apple machines anytime soon (unless you've got a skilled hacker friend and taking chances with your data).