You know, I've seen too many movies to totally write Jin off yet, in fact, I think is alive now more than I did before. 3 reasons:
1) Others here have mentioned it, but in the Lost preview show "Destiny Calls" the producers carefully say Sun "believes" Jin is dead. very deliberate and ambiguous word usage.
2) Also in the preview show there was absolutely no mention at all of Charlie. I can understand that for a number of reasons, he's dead, he is not a main character anymore, yada yada. They are very careful not to show him at all, for example when Hurley rolls the hippie bus down the hill.
This is really only important for 1 main reason, in the same preview show when mention Sun she is headlined with Jin. Instead of it being "Sun" it was "Sun and Jin". It was suspicious because they only went over a few main characters. A reunion scene with them and their daughter would make great TV.
3) Simply he is still a headliner in the opening credits. Shannon has come back several times, but wasn't a headliner. I don't think Walt has been a headliner since he took off either.
Now it makes much more sense that Jin died in the explosion, but I'm going to need to see something more conclusive, like Ghost Jin, before I'll believe he is dead.
btw, I think you guys are onto something with the D.U.I. theory, which comes first the chicken or the egg; the writing off the show or the drinking?