To clarify my prediction just a little bit more…
Jack will sacrifice himself to return the only important character who has yet to return (alive) back to the island. That man is John Locke.
And at the end of the night, we’ll see two men sitting on the beach preparing to spend the rest of their existences protecting this island. One whose faith has been rewarded and the other who has been redeemed.
… Or, we could see something completely different. Bring it on Darlton.