There's a lot less information out there so far about Microsoft's other E3 2011 announcement, which is a new Halo trilogy of games featuring the Master Chief and Cortana.
First teaser trailer for Halo 4, released at E3 in 2011 and expected for release in late 2012. The trailer seems to pick up right around the same time as the extra Legendary cutscene after the credits of Halo 3.
Microsoft chose E3 this year for two Halo-related announcements. Both have been long anticipated and the subject of much speculation and controversy.
The crew of 343 Industries takes the wraps off their first full scale Halo project, Halo: Combat Evolved Anniversary.
First trailer for the new remake of Halo 1: Halo: Combat Evolved Anniversary, which looks to be a recreation of the first game using a modern engine, with full networking features and the original game's gameplay (including the pistol).