A beta is a full version of the game that isn't the release version. It's the version where all code testing, compiling, alpha testing, and the like have already been completed and are being tested for bugs. It would completely negate the purpose of beta testing were the game not fully complete already, because then if huge content was added then you'd just have to retest the game with that content as well. Halo 3 right now seems more like the alpha phase from what you guys are saying about what is still missing. It seems like the engine is there, and it's a compiled version with a few maps, weapons, and vehicles.

I'm sure they have a larger version with a rough single player campaign already worked up and all sorts of maps that they will definitely test separtely from this publicity stunt.