As the beta date draws nearer (currently the devs are eyeballing the first week in February), you'll receive an email asking for your DxDiag and personal info. Book-Larsson says that after the data has been verified, you'll receive a beta key. He also mentions that approximately 20,000 people have registered thus far, and GamersFirst expects to make use of 6,000 to 8,000 closed beta testers (with 800 online during peak hours).
Finally, Book-Larsson hints at a unique beta tester reward as well as the fact that no pre-existing characters or accounts from the Realtime Worlds version of APB will be carried over to the beta. Hit up the official APB Reloaded blog for all of the details.