Xbox 360 Fanboy has put up a scan of the invite card you get when you purchase one of the specially-marked boxes of Crackdown that give you access to the upcoming Halo 3 public multiplayer data. It contains the answers to many of the questions that those hoping to get into the beta have been asking themselves:

• The beta will be downloaded. Obviously you need and Xbox Live Gold subscription and a fast internet connection to play at all, but it seems some users were hoping there'd be a disc to save them the download. Looks like the answer is "no". Expect about a 1Gb download from Xbox Live.
• The Crackdown disc is required. This is probably only true for those who gained access to the beta by buying Crackdown; but you have to access the Halo 3 beta through the game's menus, and the disc must be in the drive. No word yet on how it will work for those who gained access some other way. When the beta becomes available, this menu option will activate; no code is needed.
• The Halo 3 Beta will expire. The invite doesn't say how long it will last, or whether this applies to Phase One, Phase Two, or Friends & Family beta participants, but it does cleary say that the beta will end at some point and no longer be accessible, at least for Crackdown owners.

That last one is likely to apply to most beta participants; I'd warrant that Bungie will not want the full game, when released, competing on Xbox Live with the Beta, so it likely will end some time before the final release.