It looks like the
Halo 3 beta registration has begun over at
Halo 3.com. The site is getting hammered like Ben at an open bar so it might take you a while to get in. Before you get in you need to make sure you have your Gamertag and a Live Account as you'll need them to sign up for the beta (they'll have to be linked together). I'd try logging in using IE as I'm having some issues getting through with Firefox.