***** EVEN MORE LOST SPOILERS *****
Reading over some of the blogs that DR Jennifer linked to, it occurred to me that Jack's flashforward mainly indicates that he isn't coping with escaping from the island, not that he is necessarily feeling guilty of being of three to four who managed to escape. It could well be that everyone made it safely off. If I'm recalling Kate's reactions clearly, it could be construed that she has just grown weary of Jack's downward spiral. As Ben had said, "What do you have to go home to, Jack?" It looks like he spoke the truth. Jack could be doomed to suffer the same sort of post-war trauma that so many veterans experience and he's medicating his pain with drink and drugs. And in his suffereing, he believes that the one true way to relieve his pain would be to return to the island.
I hope I don't stay obsessed like this for the next nine months!