Bungie has confirmed 3 ways to get into the Halo 3 Multiplayer Beta.
We know about getting through using the website, but you can also get through with:
A purchase of the new 360 game, Crackdown with a little sticker that says: “Includes invitation to Halo 3 multiplayer beta.” will have keys and instructions to get Halo 3 beta. Crackdown comes out on February 20.
The other way is to play in the Rule of Three promo. On Feburary 1st to the 3rd a promo will be hosted by Microsoft and Bungie. All you have to do is play 3 hours of Halo 2 on Live during that time and then you will be entered in, but only with some restrictions. You will actually only get the Halo 3 beta if you are the first of 13,333 gamers who have played during this time and you also registered on the halo 3 website then you will be in the Halo 3 beta test.