After reading through many theories, I have come up with one of my own. Two smoke monsters existed on the island the whole time. One white and one black. The black one has shown itself to be Locke, the man in black, Ben's daughter, and possibly others. The white is Jacob. It only seems fair to me to believe this, seeing that the two are equal (again, just my opinion). This being true, then that means Jacob can also take the form of the dead and talk to the living, which is why we see "the ghost of Jacob" tell Hurley to take Sayid to the temple.
Now, my theory kind of branches of here, but I continue...
So Jacob has the ability leave the island, as we witness in flashbacks, and I believe he is the purpose of Hurley's insanity. Hurley doesn't see dead people, he sees sneaky Jacob taking the forms of others to try to get Hurley back to the island.
Jacob may also be Jack's father when we see him in the cabin with Claire, or when we see him in the bottom of the well when Locke turned the donkey wheel. He told Locke to tell Jack he says hello, furthering Jack's interest in going back to the island.
We know that Jacob is the reason for people coming to the island, so it seems perfectly logical that he is taking the form of the dead to manipulate the survivors to come back.
When the man in black takes the form of others its not to bring people to the island, but to further his plan of killing Jacob. Although, he may have interest in bringing back Locke because Locke, now the leader of the others, has the ability to push Ben's buttons. These pushed buttons lead to Ben stabbing the Jacob we have come to know. But is Jacob really dead??? Dun, dun, dun....
No, Jacob is not dead, nor can he be killed. The man in black can't be killed either. The are caught in a never ending game where the battle back and forth for control of the island.
In conclusion, I love some of the ideas I have flowing threw my mind, but to be honest, can this big of a twist occur? This is the final season, a season in which we are supposedly getting everything answered. Not more questions. The season finale will be a sad day in T.V. history. I feel the potential for more seasons is extremely high... too bad.