Okay, just spitballing here, but I wanted to try and condense some of the excellent ideas I've been seeing here. What if Jack the Shepard is really there to act as a guide/Lighthouse to Locke to help lead him back from the clutches of MIB? As pointed out elsewhere, it seems as though Locke is still in there somewhere and looking for a way out. I know this season is supposed to contain a big confrontation between Jack and Locke. Sounds pretty cool to me!