(Yes, I read a blog about a TV show. Regularly. Actually, I don't know how anyone can watch LOST without some sort of viewer's guide to keep up. It's like playing the original Legend of Zelda without Nintendo Power. No thanks.)
It was a nice summation of why we cannot change the past, even if we could go back in time:
"Say that you went back in time to stop John Wilkes Boothe from killing Lincoln. The whole point of you going back to this time period would be to stop the murder. However, once you stop it you would not have a reason to go back in time. Therefore the murder would still occur since you would not have gone back in time to stop it."
It made my brain hurt, but it actually seemed to make sense. I'm sure a bunch of 12 year-olds are like, "duh, we figured that one out in third grade." Well, I didn't, so back off.