So, this is just a quick theory I came up. Just remember that Jacobs TOUCH that he did on Jack and co. that were in 1977 is the key.
What if when a LOST character dies, they go to an alternate world and reunite with other dead losties, like Charlie(not saying Charlie is in S6).
So, what I'm saying is, is that when the bomb exploded Jack and co. died... only the people RIGHT THERE died. I think Radzinsky and the others got away safely, and the island HEALED itself immediately. WHH, hazard suits are used because of the radiation. Anyway, when Juliet blew up the bomb, Jack and co. died, sending them to an alternate reality where only DEAD people go(though you can still encounter other people that haven't died).
Jack and co. will suddenly "flash out" of the alternate reality for 'dead' people, and end up back in current time, which is 2007. Jacobs touch is the key.
Dalrton before the S5 finale: "This is going to be a 'touching' episode."