According to this article at CNN (which appears to originate from CNET), Microsoft is discounting the heck out of Windows 7 Home and Professional upgrades for a period of two weeks.
Starting tomorrow (June 26) thru July 11, you will be able to purchase the upgrade versions on Windows 7 Home and Professional for $40 and $99 respectively.
Those of us who are running the release candidate need to jump on this offer. Unfortunately, I haven't found any word of a discount on Windows 7 Ultimate, so those running the RC may be forced to reinstall to downgrade from Ultimate to Professional.
The upgrades should be available at Best Buy, Amazon and many other retailers. I checked Newegg.com but as of today, all they list is the release candidate.
The article also mentions something about limited quantities. They weren't able to expound on that, so who knows what that means. Limited number of copies per customer, or a limited number for the entire community?