If you couldn't register on Halo3.com the first time out, and the idea of buying Crackdown miffs you, then relax: there are still other ways to get into the Halo 3 beta to be held next spring.
The Gamerscore Blog lists two new ways that US residents can get in: by buying Crackdown, by playing at least three hours of Halo 2 online on Xbox Live and then registering at Halo3.com (first come, first served, 13,333 spots available).
Bungie.net also has an updated item on how to get into the beta test, which mentions an additional Bungie "friends and family" program that will precede the public beta.