The first-ever trailer for a new game in the Halo series, titled Halo: Reach, has been revealed at Microsoft's E3 press conference. I'm not too knowledgeable about the Halo story, but it would appear that Reach is a prequel to the original trilogy, given the trailer's initial message, "you already know the end."
The game is slated for release sometime in 2010. Additionally, Microsoft announced that those who buy the upcoming Halo 3: ODST will all receive invites to the multiplayer beta test of Halo: Reach.