How does how he "obviously" survive?
The way I saw it, he is either a) dead on the island or b) still alive on the island. I think the date on the tombstone leaves his fate wide open...
If he is still on the island or dead, then they were doing a flash forward and flash back at the same time, which I don't think they've done before. Not saying that can't be it, but I took the Jin/Sun scenes to both be set post rescue, which would mean that Jin is alive, but living a seperate life. If that's the case, either Sun doesn't know he's still alive or she can't be with him for some reason, which would make her equally sad. If they do know he's alive, seems kind of pointless to have them go to his grave.
Obviously the writers have some plan to tie all this up, but I'm starting to get impatient to actually see it!