Not sure how they'll do it but I'm fairly sure Jin is alive. I just can't see them writing the character off the show like that, especially when they set up his "death" with Sun's flashforward at his grave. Whereas Michael is definitely DEAD.
What would fit better for the format of this show is First tell us Jin is dead in the flashforward, then is audience is waiting for it to happen and see if it was true, then show the explosion making us and Sun think "there's no way he could have survived", then a few episodes into next season show Jin again, leave us wondering for an episode or so "how did he?" then reveal his miraculous survival. It would be a nice parallel with the people on the island mistakenly thinking all the boat people are dead, just as Sun mistakenly thinks Jin is dead. But if Jin did somehow make it back to the island would he then tell the people that the 6 + Desmond & Frank didn't die in the explosion?
Locke will be back next season in the island story, him bonding with the Others and finding out more of his destiny from Richard, we may also see him in some wacky Weekend at Bernie's/Little Miss Sunshine style hijinks in the off island storyline. Eventually, end of season 5 presumably, the "bad things that happened on the island" storyline will play out with Locke's death, prolly culminating with the return of Locke's body to the island in the other storyline. Terry O'Quinn will of course return to the show for the final season. How they will do this will be interesting. The three possibilities it seems to me are 1) Locke's dead body reanimates somehow. This would seem more likely if in fact the 'Richard Alpert is immortal' and 'Christian Shepard is still alive' theories turn out to be true. 2) Like the #15 rabbit, Locke has a double from another time... Jacob? This way Locke could be both dead and alive at the same time, allowing Terry O'Quinn to continue as a regular for the last season. 3) The events that lead to Locke's death are somehow changed via time travel, so Locke never dies and some, if not all, of the Oceanic 6 story never happens.
And Christian Shepard isn't Jacob.
The first time we see Christian in the cabin we also see another man, Jacob, with him, although we only see Jacob's eye. The second time we see Christian in the cabin he says "I am not Jacob". It must be an odd challenge writing for this show, leaving subtle hints and clues for some viewers to pick up on while also clearly spelling things out about who and what is going on so that other viewers can keep track of the story.