3 months to the day after Halo 2 came out is the secret date. Could it be? Is it possible, that actually they have a few more missions actually on the disk, waiting to be unlocked by live? I don't think it is out of the question, and even sounds like a my post is a train wreck thing to do. In 3 months, hype will have died down substantially. The Christmas season will be far enough behind that everybody who got it for Christmas will have had time to played through it. By unlocking another/the rest of the levels that date interest in the singleplayer (and game overall really) will go up, and they will recieve more sales. (Consequently, it would also require those who bought it release date and whose Live! subsciptions ended, due to being only 2 months free, to register, and shell out more money for Live! Sorry if I see that as a my post is a train wreck tactic, but it is.)
It does sound far fetched, but I would really dig it if something like this happened. I don't think it is too far from possible either. It would be similar to how Half Life 2 unlocked for those people who downloaded the game prior to its release. What ever the date though, I wonder...