Based on the few posts I've read about so far, there appears to be two major explanations for the two realities that we saw in "LAX". I'm posting this to see what peoples' thoughts are on these two theories and which theory (if either) they subscribe to.
One theory is that the detonation of the hydrogen bomb worked and created an alternate timeline where the island has sunk and Oceanic 815 never crashes. The reason why this alternate timeline was created was because of the paradoxical fact that had the plane never crashed, Jack would never have gone back in time to drop the hydrogen bomb on the pocket of energy. Thus, the original timeline had to stay intact for the alternate timeline to exist. Proponents of this theory argue that the hydrogen bomb destroyed the pocket of energy at the Swan station and because that pocket of energy was keeping the island afloat, this is what caused the island to sink. The reason that everything on the surface of the island is intact despite a hydrogen bomb detonation is that the pocket of energy was so volatile and located so far below the surface that it consumed the bomb's explosion, preventing it from reaching the surface. A big assumption? Perhaps. But it's not infeasible. The evidence that this theory cites includes lines from both Juliet and Sayid. Before dying, Juliet mentioned "getting coffee" and "going dutch" which are thought to be lines that she will say to Sawyer in the alternate universe. The assumption is that her mind is crossing realities as she's dying, which is why her final words (said through Miles) are "It worked." Because in the alternate timeline it did. It is also theorized that the newly awakened Sayid is the Sayid from the alternate timeline. That would explain why he looked confused and asked, "What happened?" Of course, it's possible that this is really just the Sayid we've come to know and love reacting to waking up after being drowned, but if that's the case, why not say, "I'm alive?" or "What's going on?" The next episode should shed some more light on this.
The other theory is that the hydrogen bomb didn't work and that there is only one timeline. The events in the "alternate timeline" will come about as a result of something that happens later in the season that manages to do what the hydrogen bomb could not: reset time. Essentially, the alternate timeline is a sort of flash forward in the sense that it hasn't happened to the characters yet. Proponents of this theory argue that despite the power of the energy pocket, it would be impossible for the surface of the sunken island to remain intact if the hydrogen bomb had gone off. What actually happened is unclear, but most likely, Juliet did not actually detonate the bomb (or perhaps the blast was weakened due to a defect in the nuclear core), and as a result the Swan hatch was built just like it was supposed to. Whatever happened, happened...right? The evidence for this theory is present in Jack's actions as the plane passed over the island in the flash forward. When the turbulence hit, Jack closed his eyes and gripped the armrests of his chair very tightly as if he was expecting the plane to crash. It's possible that Jack does whatever it is that ends up resetting the timeline and thus knows that the plane is supposed crash and that's why he's surprised when it doesn't. This might also explain where the wound on Jack's neck came from (the one he noticed in the bathroom just after the turbulence subsided). Jack was probably injured while resetting the timeline. It could also explain why he told Locke at the airport that "Nothing's irreversible" when he offered to fix his broken spine. Jack somehow managed to retain his memories of the island so he knows that Locke can be cured (or is supposed to be cured).
I think this just about covers the two theories, but of course it's highly possible that I left something out. Comment on which theory you believe in, propose an alternate, or suggest any other evidence that I might have missed. Right now I'm a fan of the alternate timeline theory, but that could all change by next week. Isn't LOST fun?