LOST theory
I think the "flash-sideways" timeline is not just an alternate timeline where the crash never happened. In this week's episode we see Jack, in that timeline, not knowing basic things that he would know if he were that timeline's Jack, like that he'd had an appendectomy when he was eight and when his son started playing piano. Further, several of the characters seem to vaguely recognize each other, which wouldn't have been true in an alternate timeline where the plane crash never happened. (And did Jack seem to recognize Claire's name when his mother read it from the will?)
I think we are seeing certain Losties, specifically the ones touched by Jacob, incompletely mind-wiped and installed into an alternate timeline. That's not 2004 Jack whose plane never crashed; that's 2007 Jack installed into 2004, who doesn't know everything that 2004 Jack did because that's not him, and he doesn't remember the island because those memories got erased (mostly) on his way to 2004. (What happened to 2004 Jack? Collateral damage.)
Since (by my theory) not all of the flash-sideways characters were in the plane crash (e.g. the ones who later died on the island), we should see some differences in how original and "transplanted" characters behave in that timeline. Assuming we get to see enough of the secondary characters to form an opinion, of course.

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I think I also have to amend my theory: we saw a hint that Claire has some memory of the island too (Aaron's name). So maybe all people who were on the plane and who are still alive at the time of this event I'm speculating about go to 2004, but the act is set in motion by a candidate.
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He's also made a point I've made over and over about the Others being illogical in what they hide from the Losties, which is encapsulated here:
(from http://www.nj.com/entertainment/tv/index.ssf/2010/02/lost_lighthouse_reviewing_epis.html)