(NOTE: Normally I check all the latest blog entries and related theory pages before posting my own theory, to make sure I'm not about to duplicate an already-posted theory, but there have been just waaay too many blogs to catch up on since the season finale, and I was away from my computer all weekend, so my apologies if this theory has already been mentioned.)
I know that a number of people (including me, before the season finale) theorized that the time-travelling Losties would end up causing the Incident in the process of trying to prevent it. (Miles even brought it up in the episode.) But after seeing the episode, it seems clear to me that the Incident, if defined as the accidental release of out-of-control electromagnetic energy, was not caused by any interference by the Losties, but by Radzinsky's drilling. The hydrogen bomb didn't immediately explode when Jack dropped it down the shaft, but the intense magnetic buildup occurred anyway.
Okay, so here's where the bomb does come into play, according to my latest WHH-based theory. As you saw, the initial effect of drilling into the electromagnetic pocket was very much the same thing that happened in the Swan station when the countdown timer reached zero -- heavy metallic objects from several feet away being pulled toward the source. And as a result, all of the construction equipment was destroyed. How in the world are those DHARMA workers supposed to build a whole station around the phenomenon while the electromagnetic force is that severe and continuing to expand uncontrollably? I believe the answer lies in the detonation of the hydrogen bomb at the bottom of the shaft! When Juliet detonated the bomb with a rock, it just may have temporarily neutralized the electromagnetic energy that had built up thus far, producing the same effect as whatever pushing the button did. The bomb explosion and the electromagnetic buildup actually cancel each other, which is how everyone on the surface of the island survives the explosion, and it gives the DHARMA scientists just enough time to come up with a quick solution, based on what they just observed, to periodically release the energy every 108 minutes, and the button protocol is born. And maybe the Losties, who were close to the Swan pit, get transported to 2007, which is why Richard Alpert thinks they all died in the Incident.
Juliet dies, of course, being right next to the bomb when it goes off... not like she would've survived much longer anyway in her condition. But had she not detonated the bomb, there would've been no other way to stop the electromagnetic energy from expanding, and the world would've eventually been destroyed, just as it supposedly would've if Desmond hadn't turned the failsafe key. So this means that Juliet unknowingly saved the world by sacrificing herself!