The date on the tombstone being date of the crash doesn't give us any clue to whether or not Jin is dead. None of them are talking about the island or anything and told the media that only six of them survived. It would make sense then that the date would remain the date of the crash to keep up with appearances. So that means either Sun is using the tombstone as place-holder to talk to him in her mind to keep herself sane, as other people have pointed out. I doubt Jin is in the ground there but her talking to his tombstone instead of a picture or other momento of Jin's is telling. If he was really alive still on the island it would be extremely morbid for Sun to talk to his grave. i.e. Have you ever went to a cemetery to talk to a spouse while they are on a tip somewhere or in a war or something? That would be extremely morbid. I doubt that is what Sun is doing. Jin is dead and I am sad.