Official announcement inside

Straight from the horse's mouth :
We are happy to inform you that the “Halo 3” multiplayer beta is now accessible for everyone who purchased “Crackdown”. The Microsoft team has been working on a software update, which went live shortly after 7:00pm PDT tonight, and is accessible through “Crackdown” in the Downloads section.

We sincerely apologize for the delay. We know we have a lot of fans that have been waiting for the beta for months, and even took time off of work to start early today. We really appreciate their enthusiasm, and we hope the small delay won’t damper everyone’s enjoyment of the game. Given the circumstances, we will be extending the duration of the “Halo 3” multiplayer beta from June 6 to June 10, 11:59 PM PDT.