Just spotted this over at Mac Rumors, that there are a few cases reported Mac Pro users can boot into their 64-bit Windows Vista (GO confirms XP support still limited to 32-bit on Bootcamp).
From the manual, Page 4, the message said:
Important: You must use a single full-install Windows installation disc (Service Pack 2 is required for Windows XP installations). Do not use an upgrade version of Windows and do not install an earlier version of Windows XP and attempt to update it later to SP2 or later. Use only 32-bit versions of Windows. If you have a Mac Pro introduced in late 2007 or later, you can use a 64-bit version of Windows Vista.
Also, from your Mac Pro restore disk (on newer shipped models, apparently), you should be able to find a file “BookCamp64.exe” under “Drivers\Apple\”, under Windows. There a spread/file sharing of this file on the internet whom a forum member has kindly shared with other Mac Pro user. As I have never used Boot Camp and don’t have a Mac at all, I can’t say if this “BootCamp64” will work with all other 64-bits processors used by Mac. It in understand that all Mac Pro are 64-bit capable.