The Rails 3.0 Release Candidate has been out for a few days, and it looks like it has some great goodies in store.
There's a lot to catch up on, but I suggest you start by having a look at these Rails 3 screencasts by Gregg Pollack.
Update: and now Rails 3.0 is done! - no more beta, it's really out there and ready to use. Go have fun!