There's not many 64bit drivers out there, and the 64bit Vista won't run 32bit drivers (apps yes, drivers no). And apparently it will flatly refuse to run any unsigned drivers, 64bit or not... (a.k.a. strictly MS approved only).
Added to which, the large majority of Windows apps are still, and can only be found in, 32bit versions. Although they'll run on the 64bit version of Vista, there's no performance gain.
And apparently UAC & pre-Vista era applications are unhappy bedfellows on 64bit Vista - none of the transparent registry / file redirection stuff works.
It'll be several years yet before widespread consumer adoption of 64bit Windows. My mum's still running Win98....