You can unlock the Ultimate by removing one ei.cfg file from the resources folder in the iso image for seeing the option for Home Premium as well as Pro and Ultimate. If you are signed up Technet you could also download the Home Premium beta separately before the RCs came out.
The main problem right off however was activating anything but the Ultimate builds. Meanwhile I'm trying to upgrade Vista to 7 on the Virtual PC at the moment to see how that goes.
By the way it seems to be going well so far for the 32bit RC upgrade over the 32bit Home Premium Vista!