As I speak, the Halo 3 Beta should be available. However, I have to wait until after work to go home and download it. At the same time as every other working person in the world. Sigh.
I know there are at least 2 people here who should be able to access the beta, so if you are actually playing it and can peel yourself away during a lobby wait or something, please post about your thoughts. I will live vicariously through you for the next several hours.