Its a different time line. The bomb reset everything in 1977 and now everything has changed.
To be precise it looks like we have an "Earth One/Earth Two" scenario (for lack of better term).
Earth One: "Our" Lost universe. Nothing's really changed.
Earth Two: "Parallel" Lost universe, as seen in the clip. The island was nuked to the bottom of the sea and the plane doesn't crash. This means, among other things: Locke can't walk, Rose still has cancer, Kate is probably going to prison, Charlie is also going to prison because they caught him on the plan with heroin, etc. Interesting, however, how in the parallel universe it looks as if only Jack and Locke might be better off from here on in.
As rex mentioned, the question now is will the two universes be combined...or was what we saw tonight sort of a "It's a Wonderful Life" type story, a chance to show us (on a one-time basis) what was happening on "Earth Two"?
