Apple Mac Pros now support the 64-bit version of Windows according to a post at macrumors.com.

This is noted in the installation instructions of the Mac Pros which says:

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.

However, Apple has not mentioned that these drivers will be available on the next OS X update which is 10.5.2.

Source: Mac Rumors