Well, i finally got caught up on last thursday's episode. I like a lot of thoughts posted above, and heres my 2 cents. I think Jin is dead. To me, sun seemed ot genuinely miss him and accept the fact that he's not coming back. If there was a chance she could get back to the island to either rescue him or go back to live with him, i dont think she would have been as upset.
I dont think Michael will get killed next week. They just brought him back, and it would be too cheesy to have him explain what happened and then die. The real question si what happened to Walt. Michael would do anything to ensure Walt's safety, including go back and work as a spy for ben.
Also, here's an interesting aside i picked up with Jin and Sun. I know they are Korean, but the korean (and most asian zodiac) are based off of the Chinese zodiac. Based off the gravestone, Jin and Sun were born 6 years apart, and in the chinese calendar cycle of the 12 animals, the animal opposite yours (6 years apart) is supposed to be the type of person you are least compatible with.