There seems to be two competing theories going here. 1.) you can't change history, whatever happens, happens and 2.) Maybe you can.
I am in the latter group. My theory is that you can change the past ONLY if you know what happens. Faraday's was getting close to this theory when he started talking about variables but he was missing a big part: He didn't know that he was going to be shot in the back by his mother.
Think about if he knew that fact. He could have simply walked into camp without a gun and may be there would have been a different result.
My theory is as soon as you observe the past you have already begun to change it. The problem with the Losties is that they are trying to change a past they didn't observe. For instance, Jack tried to change the past by not operating on a young ben linus thinking he would die and never grow up to put them through the ordeals that they had. If only Jack knew that by saving young Ben then he might have avoided Ben hooking up with the others.
********** "See he's organized, and he's on the ball. Never miss a day of school,and he's a underdog. Wanna learn more and more, cuz his mama taught him good. He's about to change the face of yo ghetto neighborhood." (c) Badu