In discussions with our reseller we were led to believe that the 64-bit version of Revit Arch will be implemented in the point release this year. Last time I looked, Autodesk's site had little info on the x64 version, but the partner product pages list "Revit Architecture 2008 64 bit" on the compatability for some of the add-ins.
Regarding performance, keep in mind that the 32-bit version will be running in an emulated mode in a 64-bit environment, likely giving poorer performance than in an x32 system. Once the x64 version arrives there's still little to suggest a big performance improvement in the x64 environment. It's true that obscene amounts of RAM are possible, but most of the user operations are primarily processor and display adapter intensive. Further, driver support for x64 systems in general typically is at the bottom of the list due to the very limited consumer base (note how often Autodesk updated even its 32 bit drivers, not to mention hardware mfr's). The biggest advantage of the x64 environment is for hyper-accuracy, not performance, in cad-related processes such as renderings, etc.
If you have a retail copy of Vista, a cumbersome test might be to install the x64 version (assuming your hardware is x64 compliant; both versions are on the dvd) and evaluate Revit x64 when it's available. It's probably safe to assume that both versions will share the same media (as they do with Acad2008). I'm not a licensing expert, but my understanding is that you would not be in breach of your Vista license if you were unhappy with the performance and reinstalled the x32 version of the OS over the x64 installation.
