Bungie has revealed more information on that Halo 3 expansion hinted at in the recent teaser video. They've also unveiled an extended trailer with actual characters in it.

The first minute or so of this new video is the same as the earlier teaser but then we see what those metal pods that crashed into the city of New Mombasa were: drop pods containing UNSC troopers. The player will control one of these troopers as they investigate the evacuated city. Could the action be a little more squad-based this time around, now that you're not controlling omni-soldier Master Chief?

The official title of the expansion, which will be a prequel to Halo 3, is Halo 3: Recon. Kinda generic but a title like Halo 3: Watch Out, The Covenant Is Here would've been too on-the-nose. The expansion is due in Fall 2009, apparently, meaning that the earlier rumor that it would arrive this January was false. That's a long time to wait, but it does indicate that this could be a pretty sizable expansion.