The PlayStation Blog has posted a video interview with Neil Druckman, co-designer for Uncharted 2. The interview goes into a bit of detail regarding the game's recently announced co-op mode, and even explains how things might work for the three players.
While players would be working through a lot of the single player maps, they'll find them tweaked for the cooperative experience. Neil explained how two players might have to work their way to the top of a building to open and gate, and then cover the other player as they enter the through the gate to get an artifact. I imagine there can be quite a few variations of this kind of gameplay.
We still didn't get any info on the story, if any, of the co-op mode. But rest assured we'll keep you informed of any details as they become available.