The Multiverse...It's just my little theory and I've gleaned the idea from several clues in the show, and if anyone wants to hear those specific clues send me a PM and I'll send the quotes and or pics.
But lets assume for a minute the multiverse is what is actually going on: Here's how I see it. You have muliple dimensions, say three, all running almost concurrent to each other, and on each reality, there is a flight 815. If one of the flights crash, then by course correction, the others must perish as well.
So on one reality, the plane hits turbulence, loses it's aviation instruments, turns North thinking it's south, run out of fuel, whatever, and crashes near Bali. Since all of these people are dead, the universe must course correct and get rid of the other planes to rid all of reality of the people and plane which now have no equals in the other realities. For no reason, our flight 815 hits turbulanece, equipment malfunctions and boom, starts to rip apart.
Now the Others, who realize that the island is not governed by any laws of time and space, and scientists who stumbled upon the place and learned this as well, have figured out that with current techonology and harnessing of some of the power, can actually intervene in some events. In one reality which Patchy was hooked, he sees a plane crash and lost in the pacific somewhere, alerts Jacob, who then uses his power to guide one of the remaing two realities to the spot on the island. Experiment away, do what you will to these people, they can never go back, for if they go to the wrong reality, there is a time... spasm, I don't know, I haven't had one in a while.

Somehow though, Desmond did go back because the destruction of the Swan entered him into one of those realities, right at the point where that Desmond made the choice that eventually led to the island. He has deja vu, all of that and when he diverts from a course that must happen, boom, an agent is inserted in to make sure he remains on the course to go to the island.

If Naomi isn't lying, then i feel bad for her, because the multiverse can never allow Desmond to be part of any universe again, and will take action to prevent Desmond or anyone aiding him from doing so. He will get word out to Penny, who will eventually join him, they will get married on the island, and die in the nice clothes they were married in, ther remaining friends will put a black and white rock in their pockets, signyfying yin and yang... whatever the heck that means (ok i got weak at the end here, but i'm tired) BUT, didn't the suit on Adam look like Desmond's 'interview' suit? (ok, now I'm guessing, but I could swear they were both grey)

anyone buying this crap?