The Micro Air Vehicle in Battlefield 3 is meant to be used to scout the battlefield, remotely spotting enemies. But in this video it becomes a tiny, mobile flying sniper platform.
As you can see in this video, the MAV is being used to carry snipers up to typically unreachable positions or even provide a mid-air crouching spot for picking off distant enemies. I'm a little unclear if this is still something you can do in the game or if it has since been fixed. So for now, beware the flying snipers.