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.