Let's just add that most consumer OSes out there are 32-bit for now; Vista 64-bit does exist, but not only is it bereft of drivers, WoW64 doesn't entail 100% software compatibility. Windows XP 64-bit edition doesn't really count, as it is an XP Pro version (to be precise, it's a re-dressed Windows 2003 Server 64-bit).
As far as I know, MacOS X is still 32-bit only too; however due to its roots and less varying hardware, switching it to 64-bit is hardly a problem.
In fact, right now, only systems like Linux or BSD can really make use of 64-bit processors - if only because most of their hardware drivers have free sources, and 32-bit compatibility is built right into the kernels.
You forgot about Windows 7 64bit, I'm using it currently and its fine, no driver issues at all.