Official word from the Bungie.net Forums:

"We're happy to finally announce that the issues being faced with the Halo 3 beta and Crackdown will be resolved shortly. The Microsoft team has found a solution and it's undergoing final testing now.

"Assuming the test pass goes according to plan, the fix will come in the form of a Crackdown title update within the next three to six hours.

"We understand many of you were eagerly awaiting your chance to jump into the Halo 3 Beta and this technical issue has put a damper on your plans. Thankfully the fix is incoming and we're sincerely sorry for any inconvenience this has caused. Ultimately we hope this will all soon be a distant memory once you pick up the Spartan Laser, launch yourself from a Mancannon and enjoy online shenanigans with your friends.

"As a reminder, people who already have the Halo 3 Beta are unaffected by this issue. We'll keep you posted as soon as we have more information."