In Sundown when Dogen is talking to Sayid, he mentions that Jacob gave Dogen a deal, and that MIB probably gave Sayid a something similar. But he made it seem like they were both the same person. Dogen has also mentioned that everyone has a scale of good and evil inside them, and that a darkness is growing inside of Sayid & Claire. We saw the scale in the cave with the black and white rocks, the "metaphor." What if the Island split the good and evil inside of Jacob in half, maybe to create a healthy balance of both on the island? The Smoke Monster could be Jacob's evil side, and the man we see dressed in white could be Jacob's good side. Maybe thats why the smoke monster couldn't kill Jacob, because he would have been killing himself. If someone else did it, they would just be "tipping the scale."
Thoughts? Sorry if this doesn't make sense to some people, this is my first time posting a theory... :)