During the Tokyo Games show, Microsoft showed an ‘extension’ to the Prepare to Drop trailer which showed an ODST Marine leaving the drop pod that crashed. It was also stated that this will ship in Fall 2009.
The trailer has just been uploaded to Bungie.net & presumably the marketplace too: look here.
Xbox.com gives us a few more details: Campaign will be a side story that takes place during the events leading up to Chief’s return to New Mombasa in Halo 3, Multiplayer will provide “groundbreaking multiplayer content” With Exclusive new maps & Forge options as well as Saved films, Screenshots…. Oh and 4 player co-op 🙂
EDIT: It’s now called Halo 3: ODST which makes more sense, but I’m not sure I prefer it.
LOL - The Super Intendant Labels the crashed ODST pod with "POSS. PARKING VIOLATION"
Josh Anderson's Blog 
That looks sweet! I can’t wait! I have been so busy that I really did not hear about this until recently, but that will make fall 2009 come faster! 🙂 Can’t wait for the new season of Lost also. Nice blog! You might just be able to get this excellence in blogging badge (if you were interested). Later!