This is a story from Joystiq where they sat down with Brian Intihar, Insomiac's Community Manager (and former 1Up/EGM Editor we might add), at PAX 08 last week. They asked him a bunch of questions about the upcoming Resistance 2 game and at the very end there are a few questions about the co-op part of the game.
First off he says that 8-player co-op is still in the game, but he goes on to explain that the game will dynamically adapt to the number of people playing at one time. This means you can have anywhere from 2-8 people playing and the next wave of enemies you fight will adapt to the size of your team. Pretty cool if you ask me.
He also tells us that you will be able to level up your soldiers in co-op as you go along. There is a variety of soldier types and you can level each of them up unlocking new weapons and other things. Along with this information he also confirmed that the co-op will be drop-in/drop-out and that there will be an overarching storyline to the co-op gameplay that is different than the single-player storyline.
Sounds like things are quite exciting surrounding Resistance 2. The game will be available for your Playstation 3 on November 4th, 2008. See you online!