One of the unfortunate elements missing from "Battlefield: Bad Company 2" was the lack of co-op throughout the campaign. And while we won't be seeing the campaign getting co-op anytime soon, EA is adding a new mode, called "Onslaught," which offers up the next best thing. Four-player teams will be able to take on increasingly difficult waves of AI-controlled enemies across four different multiplayer maps. The enemies aren't just limited to foot soldiers, either. Expect jeeps, tanks and a few gun boats to mess up your fun.
Based on the initial info, it seems that "Onslaught" is really designed as a training mode for multiplayer. It doesn't sound like you'll be able to rank up or unlock weapons, but the game will keep track of your best rounds and there may be a new achievement or two in there for you to unlock.
No word on a price tag, but I'd expect it requires VIP access for the download, which means, if you bought the game used, you'll need to pony up some extra dough. That's just how EA rolls these days.