Gamers.com, a site which has pages on Halo and Halo 2 as well as Bungie Soft... uh, Studios, featuring various pieces of information, has also put up a transcript of the Halo 2 trailer unveiled last week. One of the more interesting bits is this bit of text, which apparently appears on the screen:

Objective -- Save the Earth from alien invasion
Mission: In progress

... just before the MC leaps out an airlock high above the Earth only to land on a passing Covenant ship.

Now, lots of people have interpreted this to mean Halo 2 takes place on Earth, and while that's certainly possible, I'd say it's decidedly less interesting. Here's hoping the Chief hitches a ride on that cruiser out to the Covenant homeworld instead to fulfill his original mission.