It's been rumored, it's been confirmed, and now it's here. Windows 7 Release Candidate, valid for a year.
That's right, Windows 7 RC has been given an expiration date far ahead in the mists of time: June 1st, 2010, to be exact. So don't be shy about giving it a whirl.
The extra-long lifespan is to encourage people to take their time, read reviews, and not feel rushed to grab a copy before it becomes useless or expensive. There are unlimited keys for the release candidate, so don't fret, and do try it out.