This idea was kicking around in my head and think it might be pretty cool and something that we could actually find out in the bonus material of the dvd.
What if Jack become the Islands new Smoke monster? I know I am probably going to get blasted. How can you take the shows best character and turn him into something so evil as the smoke monster. Well I noticed a lot of similarities and thought huh, that could happen. Plus isn't it part of a lot of great epic stories where in order for some heroes to conquer the greatest evil, they must become what they most despise. Anyway let me break it down for you.
You don't go down there, worse than death. That point has been drilled into us over and over. By the fmother, by Jacob, even by Jack himself but it something that he had to do. Jack as far as I know didn't have any special magical properties that Desmond did. So no protection from the electromagnetism. Also Jack was in the water when the lights flipped back on. Based on the equation given to me by Lost (down the hole) = (smokey), plus there was a vacancy left by MIB.
Another interesting thing was the similarities between post hole body placement. MIB and Jacks body body end out in the forest spit out by the magic hole. Jack's body wasn't as managed as the MIB, however, if you remember Jack was lowered down and MIB most likely did a Greg Louganis into the golden light.
Finally it could have be a form of penance for Jack. Heck Jack has had a massive guilt complex about darn near everything for the entire series. Maybe he knew what he was getting into and was ready to walk(float) the island for a while. It also doesn't contradict the fact that dad Shepard told him some died before some die after, because in fact Jack died the day he was in the pool when the lights turned back on. However, maybe he was the last join the others in the church because he still doing the job as smokey.
Island needs a smokey, always has always will and how cool would it be to see in the bonus material Hurley talking to a stern Jack like smokey. Peace