Okay, so when I gave my theory about this earlier in the year I was confident that it has some backbone, but a couple of people didn't think so (see my blog http://lostpedia.wikia.com/wiki/User_blog:Daneveritt/Changing_Time:_A_Theory). Basically it talks about how Desmond was able to change time once before only by correcting it himself (with the purchasing of the engagement ring then throwing it away), this happened after the magnetic anomoly at the Swan. Which may happen to those affected near the Swan in the 70's.
Think about it, the writers wouldn't work up so much story about Desmond's 'mind travelling' if it didn't mean anything in the long run. My theory is that Jack, Hurley, Kate, etc, will travel back to random times in there life, like Desmond did, and be able to change the future, or, choose not too.
I only bring this theory back up after what has been said at Comic Con 09 that alternate timelines may come into play. I won't go into detail about it becasue I'm sure some would regard it as spoilerish if they haven't yet or don't want to hear.
I know it's a bizarre stretch, but the alternate timelines seems to give my theory more merit!...And I love my own theories :D