Speaking with G4, Naughty Dog's Evan Wells has said "So for future DLC, could we add more co-op? Probably more in the arena-style type of gameplay, like our Survival and our Gold Rush modes."
"The objective-style co-op that we have where we have those little narrative moments...it's something that we are considering and we'd really like to do, but it's a much, much bigger investment of time because we have to do a lot of motion capture with the actors and record a lot of voices. So we would absolutely want make sure that we talk to our community and make sure there was the proper demand for it."
So come 2010, you'll probably get more co-op. You may even get more story-driven co-op. Merry Christmas, Uncharted 2 fans.