Microsoft has just confirmed the speculation that the teaser trailer released by Bungie a few weeks ago is in fact a storyline expansion to Halo 3. Taking place prior to the events of the Halo 3 story, Halo 3 Recon (its official title) will put gamers in the shoes of a UNSC marine trooper -- specifically an Orbital Drop Shock Trooper -- and marks the first time since players took on the role as the Arbiter to play through the Halo universe as a character other than the Master Chief.
Halo 3 Recon, the standalone expansion featuring a new hero and new campaign, hits the Xbox 360 exclusively in Fall 2009.
[Update: According to an Xbox.com News Flash, Halo 3 Recon will take place during the events leading to the return of Master Chief to New Mombasa in Halo 3. The game will feature a brand new campaign, four player co-op gameplay and new multiplayer content. Halo 3 Recon will include exclusive new multiplayer maps and Forge options as well as maintain what gamers loved from Halo 3 including Saved Films, screenshots and campaign scoring.]