Okay, so we all know Sayid came back to life and everything, and a lot of people are speculating on the fact that he could very well be a loophole of Jacob's, similar to what MIB did with Locke. Here is why this is NOT true.
I only have one reason but it's probably sure to sway at least a few believers' opinions the other way. When MIB took the form of John Locke, he did not take John Locke's body, only his image. John Locke's body still remained. Sayid woke up, and he was obviously still in his body. If he wasn't, we would have seen his body lying around right beside him when he woke up. This is definetely only Sayid. Jacob is not taking his form whatsoever, or else there would have had to be a body lying around.