Microsoft still supports XP x64 out of legacy, but the vast majority of 3rd party software and hardware vendors have all but given up on it, and have moved onto Vista and 7 x64. Some people might not need very much by way of specific drivers or programs, but then XP x64 offers little advantage for basic computing over regular XP (or even Vista). Plus, getting 32bit programs to work reliably on XP x64 has been hit-or-miss.
Given that Vista and 7 x64 have significantly more robust (and reliable) support for 64bit (as well running 32bit apps/drivers natively in the OS), there's little reason to stick with XP x64, aside from the hassle of reinstalling the OS (or specific admin or network setups that require XP x64). My vote, in the Windows world, still goes to 7 x64; the vast OS improvements in speed and stability greatly outweigh the 2-3 hours it takes to reinstall the OS.