Nooo... don't get your hopes up just yet. Bungie is probably not going to be taking public beta-testing applications for awhile. However, after a bit of conjecture and questioning about betas in the forum, Matt had this to say:
We haven't decided how Halo testing will be handled. Any beta testing involving members of the general public will most likely be tightly controlled.There is no guarantee that Halo fansite webmasters or any other webmasters will get a beta.Handing out betas to people who didn't really merit them was a problem with Myth II that we're trying to avoid in the future. The jump in a garbage can for a beta bit was done without the knowledge or approval of our current QA guy, and he's not especially happy about it. Don't expect any repeats.
Bungie is watching.