I think we have all concluded that the cabin resident in Man Behind the curtain is not Jacob, but the MIB, judging by the ash, the fact that he was invisible (we've seen FLocke disappear into thin air), his hatred of technology (a.k.a. progress), asking his loophole Locke to help him, and the fact that "Christian" was seen in the cabin on multiple occasions. One can assume that the ash was trapping MIB inside or maybe keeping him out to protect Jacob. I don't know. But the whole cabin thing, except for a few sketchy details (like why did Jacob live there in the first place, who broke the circle of ash, and why did Ben think he lived there but Richard knew he lived in the statue), has been given closure. EXCEPT for two reasons: what was the point of the ash if MIB came and went as he pleased and why did two people appear to Hurley in the cabin in The beginning of the End?