This year at Gamescom we were able to see the first footage from the co-op gameplay in Battlefield 3. It is pretty exciting, showing us a very nice, action-packed level where the two players working as a team take enemies out either with stealth or by making a complete mess.
In the Gamescom Battlefield 3 gameplay video below, we can also see one player using the night vision and the other using a med kit to help his fallen friend. All in all, a very nice display!
Here is the Battlefield 3 Co-op gameplay trailer (starts at around minute 2:20):