Here is my theory.
Like Faraday said, people are the "variables" in the equation that is time. In the case of out Losties, the events seem for fall in place according to the constants of the equation. Everything happens as it is supposed to right up until the point where Juliet chooses to hit the hydrogen bomb.
Everything and everyone is a constant up until this moment. The final seconds of the season finale. For those who are going to argue that time changes from the instant the Losties time traveled, think about this example: Dr. Chang's arm. His arm is not lost until the final gunfight at the swan station.
So, whatever happened, happened UNTIL Juliet detonates the bomb.
At this point there are two timelines. One in which everything happens just as we know it. The bomb did not detonate and Juliet dies from here injuries in the hole she fell in. Jack, Kate, Sawyer and everyone else continue to live in 1977.
At this point no one knows what to do since the final plan does not work. DHARMA is pissed. They capture our Losties and choose to execute them. (Proof: Richard tells Sun in "Follow the Leader" that he watched all of them die) Rose and Bernard continue to live in 1977 and live out their lives and die in the caves becoming Adam an Eve. No idea where those black and white stones come from.
Everything else continues as planned in this timeline. DHARMA clogs up the energy in the swan station and forces Radzinsky, that jerk, to work underground in our hatch and push is own button every 108 minutes. 815 crashes, the swan station implodes, everyone we know as dead is dead (or so it seems?), the Oceanic 6 are rescued, and all the time traveling continues as we know it did. And this leads the way into 316 crashing on the island and the 2007 storyline from season 5 to continue.
Then there is the alternate reality time line that everyone hypothesizes about. In this one, the island blows up and everyone on it is dead. Our Losties, DHARMA, the Others, including Ben, Richard, Eloise, and Charles are all dead. Faraday is never born. Eloise Hawking is dead and thus is not there to stop Desmond from buying the engagement ring for Penny. Desmond never crashes on the island. And since the swan station no longer exists and its energy has already been discharged, 815 never crashes.
But where would the fun in this time line be? Here are my theories. Desmond is the key. He exists independent of time and has all the memories from both realities. It is up to him to get the Losties back to the island. Maybe they use the Lighthouse station to locate and get back to the island one last time? And once our Losties return to the island, everything and everyone's memory is restored and the alternate reality ceases to exist (or rather is just pushed aside since it is no longer relevant). Everything from here is up in the air.
What does everyone think?