It appears that the actual game disc will have the code on it to get the Beta.
From
Halo3.com:
Crackdown
When Crackdown hits store shelves (Feb 20 - North America, Feb 22 - Asian territories, Feb 24 - European markets), and you still haven't secured a place in the Halo 3 multiplayer public beta you can still get in via specially marked boxes of Crackdown. These boxes of Crackdown include an invite to participate in the coveted Halo 3 multiplayer beta program when it becomes available. When the Halo 3 multiplayer beta surfaces in spring 2007, those owners of Crackdown that have access to an Xbox 360 Pro console or an Xbox 360 Core console with hard drive and a valid Xbox Live Gold subscription
simply need to load their copy of Crackdown into Xbox 360 and use the disc as a key to download the beta from Xbox Live Marketplace and join the battle in Halo 3 multiplayer.
While I'm feelin curzy... how do you interpret this?
The Rule of Three Program
The "Rule of Three" program is currently planned for gamers (17 years and older) in the 50 United States (includes District of Columbia) and Canada who have access to an Xbox 360 Pro console or an Xbox 360 Core console with hard drive, a valid Xbox Live Gold subscription and the Halo 2 game.
It is a two-step process:
1. Participate in at least three hours of Halo 2 multiplayer sessions on your Xbox 360 console via Xbox Live during the three (3) day period beginning February 1st at 12:01 am EST and ending February 3rd at 11:59 pm EST.
2. After the three day window is over, head back over to www.halo3.com on February 5th and register.
If you are one of the FIRST 13,333 to do this (and meet the above requirements) you will receive an invite notification in February to participate in the Halo 3 Beta.
To me it sounds like you can't register until February 5th, so the logjam will be trying to register at 12:01 on Feb. 5th, not racing to register once you have completed 3 hours of Halo 2 matchmaking. At least that gives you a chance to catch your breath during those 3 days.