Okay, the videos from Comic Con are out on the Internets. I’m assuming that nobody on the forum is going to want to treat these as spoilers but if for some reason you want to avoid all knowledge of the Lost panel at Comic Con until 2010, you should probably stop reading this thread now.
A few observations/theories: Regardless of how canon these videos turn out to be, it seems pretty clear that Lost is going in the direction of an alternate reality for season 6. But given that every season up to now there’s been some major new dimension added to the story (i.e. the Dharma Stations in S2, the Others in S3, the O6 getting off the island in S4, and time travel in S5), going into an alternate reality doesn’t really seem out of line as “the new time travel” for season 6. I think it’s pretty heavily implied that setting off Jughead is what will wind up spawning the new timeline, just like Ben turning the wheel at the end of S4 was what instantiated the time travel. This fits with how they give us nothing to suggest anything before 1977 is changed in the new timeline since Kate and Hugo were both born in '77 and Oceanic only had a perfect safety record since '79.
But the question of course remains, what’s different in the new timeline and why?
Damon and Carlton probably realize that going into an alternate reality that at least has the appearance of negating what happened in the first five seasons will seriously piss off a lot of fans. Thus, I think it’s very likely that they have some twist up their sleeves that will somehow merge the two timelines together, hence them saying “trust us” about the alternate timeline. If I had to take a guess, the twist could somehow involve destiny steering the alternate timeline right back to the original one right around when Faux Locke kills Jacob on the present day island. Think of it like how Desmond kept trying to save Charlie at the end of season 3 only to be repeatedly thwarted – the losties in 1977 can set off a nuke to change the timeline in a way they want, but eventually they’ll get steered back to where they would have been had they not tried to change anything.