This is a Lost post; I would advise strongly against reading under the LJ-cut if you have not seen the entire series so far (including tonight's episode).
So tonight's Lost was another bombshell episode. There have been hints throughout the seasons, really since episode 1, that the show might have something to do with either time travel or some time-travel related phenomenon. This episode provided by far the most blatant SF-style time travel theorizing; with this concept of people's consciousnesses "jumping" back and forth in time.
I'm a little worried. Often I find time travel stories to be full of holes and not very fulfilling -- Heroes being a very good example. On the other hand, there are exceptions -- the DS9 episodes "The Visitor" and "Children of Time" are extremely good and both deal with time-travel.
I have faith in the creators of the show; I think they'll find a way to pull this off because it seems like they've actually put thought into it. Of course, I won't know for another three years. :)