Well I got my Win 7 Home Premium upgrade disc in the mail today. Booted up into my Win 7 RC 64 bit OS and inserted the disc. It took me through setup and I chose a custom install and it did it's thing. It made a Windows.old folder with my stuff in it (if anything else is in the root of your c drive it leaves it there) and finished with Windows 7 Home Premium installed and running great so yes you can use the upgrade disc over top of the RC (although it isn't upgrading , it is doing a clean custom install) Took about 25 minutes total.
It went a lot easier than I thought and right now I'm getting ready to use the built in maintenance tool in Win 7 to do a backup image of the c drive so if I ever need to reinstall it I can just reimage the disc.
I liked the RC before but damn this final version seems even quicker, I like it