If you've been playing in the Beta since the 11th, you'll have to install an update today to keep playing. If you don't, you might get a message that says the H3 Beta is "unavailable or expired".

You should be prompted to download an update. If it doesn't happen, quit Halo 3 and/or restart your 360 until you get the prompt.

Just a warning-- you will NOT be able to watch any of your old saved films after you do this update, but you will NOT be able to continue playing in the Beta until you do install the update. So if you've got any movies that you're especially fond of, watch them now, then update.