UPDATE: No. Most of Halo 3's features are known at this point. Bots are not in the game.

This always gets asked. Bungie hasn't ever done a multiplayer game with bots. It's conceivable, I suppose, but not at all likely.

UPDATE: For clarity, Bungie games have featured enemy AI during multiplayer matches. Similar to the original Doom shooter, Marathon games could feature aliens in multiplayer matches if desired. However, Bungie has never made a game with multiplayer "bots" that play in multiple competition styles (slayer, team slayer, capture the flag) that were popularized by shooters such as Quake, Unreal Tournament, and others.