Just watch - I'm pretty speechless.
UPDATE - Multiple sources state that this project is a DLC extension to Halo 3 - essentially 'another act' in the campaign to download. The Halo story arc hasn't finished yet! Rumoured date for release is 06/01/09.
UPDATE 2 - Rumours say that this is indeed a sequel to Halo 3 (probably connected to the planet seen at the end of the Halo 3 cliffhanger) but is a squad based shooter (similar to Gears of War). Interestingly, it seems that this won't be DLC, but a whole new retail title. A Bungie spokesperson said to IGN - "We're always looking at ways to further expand and improve the Halo 3 experience, but there are currently no plans for an update to add new campaign content."