The new Intel-based Macs will not run Windows XP, because current versions of Windows do not support the new EFI BIOS that the new Macs use.
x64 versions of Windows and Windows Vista do support EFI, unfortunately Vista won’t be out until the end of 2006 (maybe) and current Macs use 32-bit Intel processors, so they are unable to run the x64 version of Windows XP.