Ok, this is what I think. The bomb is necessary for the time travel thing, as we remember in “Back to the future” the key to time travel was to have a source of radiation (like the plutonium or in “Lost” case a hydrogen bomb) for the nuclear reaction to generate the power require mixed it with electromagnetism or something (that later on conceive the idea of the flux capacitor). Anyways, the thing is that we have the recipe for time-traveling now. That’s the evidence to assume that our Losties are going to travel in time once again.
But I believe the only ones that are going to travel are the ones Jacob touched. We never saw Jacob touching Juliet or Miles (unless they show it in this season). So, it was bound to happened, they were in 77’ because they were suppose to, so they can be on the island and play the game of Good vs. Evil. Their presence with Dharma was necessary so the future events could happen. They could not change their past or the history of the island; they are making sure it happens.
When Jacob said “They’re coming” he is telling his Nemesis that the joke’s on him, Jacob found his own loophole too. The Losties are coming to their right timeline to make sure that Good is going to win the War/Game. What do you think? Any other ideas?