Sorry if this has already been posted, I've had little time to keep up with the posts.
But, waiting for the last season and the premiere (LA X), I can't help but fear that they really altered the time line. To me, there are lots of evidence that they didn't. I'd like to know what you guys think.
First off, Eloise reminded Widmore that it was also difficult for her to have sent Daniel back to the Island, "knowing" what was going to happen: she called it a sacrifice. (I can't recall which episode was this, I think it was "The Variable"). She was quite emotive. Later, we find out that she accidentally killed her own son. So, if by traveling back in time, one can alter the time line, Daniel's very presence on the island in the 1970s would be an alteration. And if this is true, in the original timeline, Eloise never killed Daniel in the 1970s, because he had never shown up. In the alternative timeline, she did. But how come this only happens in the alternative timeline if, in the original one, she said that it was difficult to send Daniel to the island? She obsviously knew something WAS going to happen to him. Also, when she and Widmore try and convince Daniel to go to the Island, I think they were expecting him to blow the up the jug head, because this IS what happened in the original timeline. Whatever happened, happened. So, the whole Eloise convincing Daniel to dedicate himself to his mathematical abilities was just a way of guaranteeing that he would become a brilliant physicist and would fulfill his role back in the 1970s. So, in a way, just like Eloise said to Widmore, Daniel "had" to be sacrificed (killed by his mother) in order to everything to play out.
What's more, Miles said "What if your buddies are gonna cause what they're trying to prevent?". I think that's exactly what happened. The jug head was the incident. And the whole "infection" on the island story (check out season 2, the Claire kidnapping and stuff) could be a result of radiation or something, due to the H bomb explosion. Also, "when Sayid first visited the Swan he tried to find a way past the concrete wall, but it was too thick to get through. Sayid told Jack that "The last time I heard of concrete being poured over everything in this way was Chernobyl."" (from Lostpedia). Wow, how about this? They had to seal off the Swan, probably to conceal the leak of radiation (left by the H bomb explosion) and/or electromagnetic energy.
Well, there are more things I've noticed, but my post is big enough for now! Hehe. See you!