Halo's Grifball mode tasks teams of two with trekking a bomb to their opponents' base, and it's been a staple for some time now. It's getting a significant upgrade in Halo 4, though.
It's now a separate, full-fledged game mode, meaning you can tweak it to your heart's content, and there are some major changes being implemented. For example, teammates can now throw the bomb to one another, and killing a bomb-carrier with the sword puts the bomb directly in your hands, though doing it with the hammer still sends it flying.
Check out the Grifball announcement at PAX above. Are you a Grifball fanatic?