Jack is not Jack by Alex

There two entities in the island.
One has the form of Locke and the other has no form...yet.

So, how this is going to play. I think some of the losties are going to die and the rest of them will be badly cornered. (probably by FLocke)

At that point, Jack will be presented to make a choice. Either not jump in and watch all die, or jump in and let Jacob take his form in ATL and Jack stays to play Jacob's role on the island and everyone else (but Jack) gets a second chance in ATL.

This switch probably happens on 815 flight (in ATL time), probably when he grabs the seat really tight. Kind of explains the funny looking mirror recognition phase. Also, Jacob (in Jack's form) has mixed memories of his appendicectomy, which he solves by asking "his" mom.

Extra cool would be one last time travel for Jack in ATL (a year or two ahead) just to make sure that all of the people are having a nice life. Probably won't happen, so I guess he'd have to take Jacob's word. (I could live with a heartbreaking goodbye to Kate in ATL future though :P)

So, that leaves us with Jack in Jacob's role in the island and Lock in MIB's role.

The only thing that doesn't add up is that Locke always wanted to stay and be the leader of the island and Jack always wanted to leave, which is kind of reversed in roles.