I think Ben has imprisoned Jacob. When Locke was investigating the circle of sand (?) Ben stopped him. I don't think Jacob is Ben's personal smoke monster but I think Jacob controls the smoke monster. It is his out of body apparition thingy. He certainly is capable of violent outbursts, enough to say kill a pilot and chase Juliet and Kate through the jungle. And has wierd mind powers, he created fire and broke windows! What I thought was interesting was how Ben was shaking at the suggestion that Locke get to meet Jacob. Nervous what would happen between Locke and Jacob? Did he realize this is his chance to kill Locke? Or is he just plain scared of Jacob? Once he gets to Jacob's hut he didn't seem afraid of him. I think he has Jacob restrained somehow. Also very interesting is that Ben was told to kill his father (I assume) and Locke was asked to kill his father. Maybe smokey brought Cooper to the island. And it was a test for Locke by Jacob. But Locke couldn't kill his father and Ben had no problem with it. Ben=Bad Locke=Good