So from reading a lot of the posts and various theories on this site it seems to me like there is a split among the fan community. Some people believe that, like Daniel says, you can't change the past and whatever happened happened. Others seem to believe that the past can be changed and the survivors of 815 are currently changing the past by what they do in Dharmaville in the 1970's. I have always believed that what we are seeing now in the show did actually happen in 1970's. We are just seeing it from the 815ers as they are experiencing it now. I think that Ethan's birth proves this to be true.
Oceanic 815 crashes in 2004 and it is then that we meet Ethan. Now if Sawyer and Juliet never saved Amy in 1974 then Ethan never would have been born. They had to have saved Amy in 1974 for Ethan to ever exist in 2004. So they are not changing the past at all. I hope this helps some people realize that the Sawyer group and anything they do from here on in the 1970's is not changing the past. As far as we know the only character that is capable of changing the past, or having his past changed, is Desmond (most likely due to his exposure to the electromagnetic anomily when he turned the key at the swan).