I installed Windows Vista Business x64 (from MSDNAA) on my new computer on 2008-07-19.
This was, of course, one of the first things I wanted to do. And this is actually something that seems very much improved from XP.
Apparently all you have to do (according to this article) is move the folders to the new location. As in, go to the Start menu, click your username, select all, right-click drag the folders to the new location, and select move. If anything referred directly to the C: location, I suppose that would need to be changed.
Also, it doesn't seem to apply to the hidden AppData folder.