I am building on *several theories* of others but I have something I would like to add to the discussion.
It has been argued by others that:
*Every season of lost has started by showing us how the flashes of each season end. Season 1 flashbacks show how they end up on the beach, Season 2 flashbacks show how Desmond ends up in the hatch, Season 3 flashbacks show who the others are, Season 4 flashforwards show how Jack concludes "We have to go back", the Season 5 time flashes show how Daniel was at the Swan construction site and met Dr. Chang.
*Season 6 will be the same. The first thing we saw was how the season ends. This time there is a new story telling device. Now we are seeing flashforwards of flashbacks. The off island LA X stuff IS NOT an alternate reality created by Jughead. We are seeing the reset timeline that has not been created yet.
*Jack looks older in the season 6 premiere. That's because he is.
*In the same way that Charlotte mumbled about chocolate when she was dying, Juliet talked about coffee and going dutch. Her mind is flashing to another period of time... I think in the off island scenes, we are going to see a lot of character resolutions. Juliet will meet Sawyer, they will flirt with each other, and she will ask him if he would like to go get a cup of coffee, for instance.
Basically the idea is that the off island alternate we are seeing right now is going to be the result of what happens on the island this season... not the result of Juliet hitting Jughead.
What I want to add to this theory is this: I think I know how Jack will reset everything AND how the island ends up at the bottom of the ocean.
We know this is the final season and the writers have promised us that all of the major questions that are important to the characters will be answered. One major question that is of obvious importance to the Losties is how the island can move through both time and space.
I predict that at some point in this season, probably somewhere around episode 12-14, Jack will learn what the island is and how it can be moved and the nature of time travel. In the mean time, I predict many of the characters on Lost will die. In fact, I would not be surprised if by the end Jack is the only one still living.
I predict that Jack, the man obsessed with fixing everything all the time, will come to the conclusion that blowing up the swan's pocket of energy was not sufficient to change everything. I predict the major battle we are all predicting for season 6 between "Good" and "Evil" will involve Jack deciding that the island itself has to be destroyed.
Jack is going to move the island. This time he will move it to a place where it can't possibly cause 815 to crash. He will move it to the bottom of the ocean (space) and at some point around 1970 (time).
This explains why the island was at the bottom of the ocean, yet everything seemed to be basically normal there. It didn't look like a nuke had exploded. It didn't look like a volcano had erupted. It simply looks like it ended up there.
I predict it was placed there around 1970 because I think Ben will be seen off island, which means he never made it to the island in 1973. Yet the New Otherton was seen, so it was moved to a period in time after The Dharma Initiative arrived.
Because Jack moves the island to the ocean floor at this point in time, all of the people Jack cares for will be saved, including Juliet since she will never go to the island. Heck, he might even realize he can save Ben by ensuring he never goes there and is never shot by Sayid. Heck, Ben might even be a decent guy when we meet him off island.
The show is going to end with Jack, who's neck was scratched up in the final battle, on Flight 815 headed to LAX. At this point, I think Jack remembers everything that happened. That is why he was frightened this time at the turbulance, That is why he was looking at everyone so curiously and why he looked confused when we first saw him. I think he asked Desmond if he knew him simply to try to gauge whether or not he was the only one who remembers what happened.
This is also why Jack told Locke, "Nothing is irreversible."
The off island flashes we see this season will turn out to be the true ending of Lost. Everything else was just progress.