I just had a thought, i've used this method to install build 7600(don't ask me where i got the key) over 7264(and a few earlier builds before that) but all of these were the Ultimate version. I think this upgrade method doesn't work if you want to install the Home version over the RC(RC is Ultimate), really not sure about the Professional version ... So if you get Win 7 Home i think you'll have to do a complete reinstall. I'm gonna do a reinstall anyways after i get my hands on a legal copy of Win 7 rtm, cause i really want to do it "clean" not to mention i'm a "pirate" right now and don't feel comfortable with that. Back when i first installed the RC i just did an upgrade from Vista to Win 7 RC and then a few more upgrades as more RC builds were available. Not saying that there's something broken over here but i really think a complete reinstall is the best option to avoid any potential issues that may arise with all this upgrading i did.
Cheers !