Some people are saying they think in the final bomb scene, if you look at the last few frames, it appears to be Jack waking up again on the island point-of-view seen the last two times it has happened. This had lead some to think that maybe when the bomb went off it reset things, but only back to the O6 first appearing on the island. If so, it could mean they now are not only ALL alive again, but may also have the knowledge of what happened before and now can correct the mistakes they did before and resolve it through to the end in a proper manor this time. Sort of like the bomb made it so the first time through was like a dry run and now they can do it again the right way. I'm not so sure about that, but it is an interesting theory and would be a way to get everyone back to go straight to the heart of things for the final season.