The process works exactly as it has in the past--you go to a , sign in with your Windows Live ID, answer some questions about your internet connection and other Xbox Live-relevant information, and pray to the beta gods (having a Xbox Live Gold account helps). If you miss out, you can still expect to see a roundup of the new features on GamePro a few days before the public beta starts (much like ), as well as a handful of Xbox Live tweets from yours truly.
You can find out more about the Xbox Live Preview Program via Larry "Major Nelson" Hyrb's , which is conspicuously devoid of Last.fm mentions. Hopefully it'll still be part of the next update--I've sent word to Major Nelson himself asking about that.