Sure, we all watched him seemingly fall to his death, but the golden rule of death in movies is if you don't actually see death happen, he ain't dead. When Cap finds Bucky during the rescue mission, he's strapped down underneath some pretty fancy looking equipment and the way he's mumbling to himself suggests this may be some sort of hypnosis machine, which would allow for the brainwashing he's given to become Winter Soldier. My theory is that he's alive, but I don't know how/if this will become relevant in further Cap sequels. My thoughts would be that in Cap 3, Bucky would resurface as Winter Soldier alongside whatever the main plot is. Also, there's the one big gap... to play a part in future sequels, he not only has to survive the fall, but somehow stop aging for about 70 years. That one I don't get how they'll pull off again. Theories?