Details were just released about the Halo 3 beta, specifically the date. The beta will kick off on May 16th and run until June 6th. Plus Bungie posted a new MP Video with (to quote Frankie) ‘a ton of stuff…you simply haven’t seen before.’
Posted from the Providence rail station
Edit: I am being told it is also on the Marketplace, but I can’t confirm that since I am not near my console.
Edit: I have confirmed this is now on Marketplace for all regions. The trailer is in SD and weighs in at about 348 MB.