"Splinter Cell: Chaos Theory" had some fantastic multiplayer modes, and it looks like "Splinter Cell: Conviction" will be taking a nod from that game in terms of the co-op. Ubisoft has released info on the mode, as well as a trailer, and it is looking decidedly excellent.
The co-op campaign in "Conviction" will take place before the events of the main game, placing two players in control of two spies, one American and one Russian. Since they're both spies you're probably not going to fare too well charging into a room, guns blazing (though the trailer does feature exactly that). If it's anything like the co-op in "Chaos Theory," you'll have to use stealth and diversion techniques to get the drop on enemies.
Ubisoft has said that the co-op campaign lasts about 3 hours, but I'm sure there's plenty of replay in that, with higher difficulties and such. Plus, there's always the potential for DLC.
Anyway, here's the trailer Ubisoft released: