I've seen the trailer and was able to play the game myself at the GC in leipzig today. But the lads over at The chaps over at IGN got some hands-on time with Call of Duty: World at War's four-player co-operative action and where able to snag video from it:
The objective is to take out three antiaircraft guns atop some concrete fortifications. The easiest way to take them out is the bazooka; just stand off at a distance, get a clear shot, and avoid having to take out the mess of infantry between you and it. However, you can blow them with charges if you're short of rockets. Then comes an Japanese counter-attack, so you have to man some machine guns and take up positions on top of the fortifications.
This was a co-op battle, so there was naturally a lot of chatter going on about who should go where and who should shoot what. Apart from that, the campaign is the same whether you're playing solo or playing with three others, though the difficulty does scale in relation to the number of players in the battle. If someone is killed, another player can revive the fallen but if everyone dies at the same time, it's back to the previous checkpoint. That's if you're playing regular campaign co-op. There's a nifty little twist called competitive co-op that basically adds a scoring mode; you get points for kills, extra points for head shots, etc. The winner, of course, is the one with the most points. It's possible to really rack up a high score if you manage to snag score modifiers that multiply your points. The kicker is that in competitive co-op, if you die, you lose points, and if someone revives you, they earn points. That mechanic is designed to reign in players, because if you charge ahead or take a lot of risks or play wildly and get killed, you lose points.