While Gears of War has had co-op since the start, the series will be doubling the amount of players who can participate.
According to Cliffy B, Gears of War 3 will support up to four players in the main campaign. Each character will also be a part of the story, so no worries about just being dragged along for the ride.
I’m going to take a shot that this feature may be online only, but I could be wrong. But honestly, how many retail releases on 360 and PS3 support four player split screen nowadays?