Woo-hoo!! Halo 3 was finally officially announced on Bungie.net today, and there is a little trailer for it. It doens't show much, but it does show Cortana coming in all scrambled, apparently on a different Galaxy. Master Cheif in on Earth, and it is being overrun by the Covenant. No new enemies, weapons, vehicles, or anything has been announced yet.
All we know is that it is planned on being released in 2007. Not even sure which quarter at this point?