Well, they didn't really hone in on co-op, but they did show off a couple things.
First up was the 8-player co-op campaign enabled Resistance 2 coming in November. They didn't show the co-op or talk about it, but we know it's there.
The other game was a new one called MAG: Massive Action Game from Zipper Interactive, makers of the old SOCOM games. This game will have 256 players able to play at the same time and you play with a squad of 8 members and the more respected members will be the leader of that squad. Only a pre-rendered movie was shown ala Killzone 2 a few years ago. It sounds pretty cool and Jack Tretton, President of Sony America, said that it "pushes the PS3 to its limit". Translation: this is a long way from being done.