Microsoft, today, has announced two new ways to get into the Halo 3 Beta in case you missed your chance. The first is to buy a copy of Crackdown, which will be released in the US on February 20. It has been said that the Crackdown disc itself will be the key to allow players to download the beta from XBL. The other, referred to as the 'Rule of Three', involves players playing three hours of Halo 2 multiplayer sessions over XBL between February 1-3, and then register at halo3.com
It looks like there is still a ticket into the beta. I think it's kind of cheesy, however, to get copies of your new game to sell by offering the chance to play the beta of another game, lol, but I guess whatever works, right?