Well, this will happen sooner rather than later: “We’re weeks rather than months away from starting closed beta,” said community officer Chris Collins to Eurogamer during the PAX in Seattle.
All Point Bulletin, which is currently slated for an “early 2010” release, accepts beta applications on the official website, where you will have to answer quite a few questions (but are not obliged to answer them all) and hope that you’ll be picked up. Good luck!