People already testing 3.7 will be automatically updated to RC2, which is convenient. And for those of you who would like to start testing now, you can try the WordPress Beta Tester plugin, or download the the zip of release candidate from Nacin’s post. He asks that testers please post to the Alpha/Beta area in the support forums to report bugs. You’ll also be able to see any known issues here.
For more on 3.7, you can reference the announcement post for Release Candidate 1, which was published October 18.