According to the Halo Blog, if you played Reach last night or this morning, you were probably prompted to download the Title Update.
“Please enjoy the sample TU gametype, but be aware that it is not representative of the final Matchmaking implementation. You may find a few discrepancies in this one-off gametype, but hey, that’s why it’s a beta. We will be looking at data from custom games, Matchmaking and voting for the next few months, so pardon our dust. One more difficult issue is that the very rare Halo: Reach “Preview Disc” (a tiny amount of these discs were given to certain community members and it has nothing to do with either Games on Demand versions or normal retail discs) cannot be updated with a conventional TU. 343 is hard at work trying to come up with a solution for that small, but understandably vocal group of people.”