I see everybody is disscussing if Jacob's Nemesis or MIB is a shapeshifter. I guess this is because of the "loophole" dialog in the season 5 finale. Some even thinks that he is related to smoke monster.
Why noone ever thinks that MIB might be smoke monster itself? Maybe MIB was another dead man on the island, and embodied by smoke monster to talk to Jacob. There is no such man as Jacob's Nemesis, I think. It's just the old Smokey everywhere.