Bungie today released a teaser for one of their projects. The video showed New Mombasa being hit by tons of Covenant drop pods. The camers flickers and and the text, "Halo 3, Prepare to drop," appears. Perhaps a Halo 3 expansion? Only Bungie knows.....
Edit: Upon more in depth viewing an discussion in IRC it appears as though this game may be and alternate campaign from Halo 3. Perhaps one where you play as ODST. If you study the video carefully you can see that this could be the scene in Halo 2 where the Covenant ship makes a jump while inside the city. During which ODST pods are being dropped, not Covenant.
Download trailer WMV