Surprise, surprise! Microsoft has announced that they will be accepting signups for the upcoming Halo 3 beta this Monday, December 4th. This will only be available to gamers in North America, and only a select amount of registrants will be accepted.
From GamerscoreBlog
The Xbox Live multiplayer public beta, which is a pre-release version of the multiplayer experience of “Halo 3,” is scheduled for availability in spring 2007 exclusively on Xbox 360. This beta also represents an opportunity for gamers to participate in the development of “Halo 3.” Through the resulting feedback, Bungie Studios will be able to further refine and hone the end result of “Halo 3.”
You can sign up at http://www.halo3.com