Microsoft and Bungie, in case you were unaware, announced Halo 3 yesterday at Microsoft's E3 press conference. At the conference, attendees were treated to a teaser trailer for the sequel, which we've uploaded here for your viewing pleasure! Halo 3 screenshots have also been released, and you can get an eyeful of those here too!
Master Chief returns to battle on Xbox 360 in 2007, Bungie revealing that the sequel picks up where Halo 2 left off: "The Master Chief is returning to Earth to finish the fight. The Covenant occupation of Earth has uncovered a massive and ancient object beneath the African sands - an object who's secrets have yet to be revealed. Earth's forces are battered and beaten. The Master Chief's AI companion Cortana is still trapped in the clutches of the Gravemind - a horrifying Flood intelligence, and a civil war is raging in the heart of the Covenant. This is how the world ends..."