GameDreamZ has a story up, rumoring that Halo 2 will have a multiplayer mode where Spartans face off against the Covenant in missions that will tie in to the single player story arc-- very similar to what many old school Bungie fans predicted (or rather hoped for) in the original game:

First up is the slickest of the news bits, Halo 2 will have an online mode called Covenant vs. Humans. As it sounds, this mode pits one side of players, playing Elites, against their opponents, the Spartans! Even more drool worthy, is the news that these online missions will tie in directly to the single player story. I'm not exactly sure what that means, but my own theory is that these online missions will move in tandem with the Chief's storyline. So gamers online will be fighting their way to the Covenant Cruiser in Mombassa as the Spartans, while the Elites try to halt their advance!

Of course, this might be nothing more than a scurrilous rumor. But if it's true, it's great news. Thanks to Break Point Halo for finding this.