Well, until I get more "info" be it from the upcoming DVD/Blu-ray release of Season 6/complete series, if any at all, my personal stance is that the island's power is what creates the afterlife for all beings on the planet. May sound silly, but the way it works in my head makes sense. Eloise makes those cryptic comments about how, if they fail to go back to the island and accomplish whatever goal, that it will "end badly for everyone" or whatever. As in, the end of someone's life if the light on the island goes out, their soul will have nowhere to go when they die.
Desmond gets exposed to a load of electromagnetism, which the light of the island seems to give off. I know this isn't really Lost-related, however, in a lot of those ghost shows you watch on TV, they have equipment for detecting electromagnetism because it's a general belief that paranormal activity is the strongest when EM levels are high or whatever, and on the island there's a big thing about ghosts, Michael saying that the whispers are souls that cannot move on, etc. Anyway, Desmond gets exposed to a lot of the stuff and it gives him the ability, later, to jump between the island timeline and the afterlife timeline in the flash-sideways. This tells me that the electromagnetism, the island's light power, and the afterlife are all related. They practically spelled it out that because of his exposure to the EM, he is consciously able to jump into afterlife-timeline. See, until The End, we didn't know the flash-sideways was the afterlife. We simply thought it was another timeline they created due to the hydrogen bomb going off and therefore it was still the same kind of thing we saw Desmond doing in earlier seasons. And since its the same energy that allows Desmond to hop-skip between realities, I believe it's all related.
So until further evidence is revealed that suggests otherwise, I'm saying the light of the island is what "generates" an afterlife for everyone-- I would say it generates the afterlife for people who die on the island, and it could very well be, but someone suggested that David (Jack's lusty son) was in the flash-sideways as Jack's son because he needed Jack and Juliet to be his parents in that reality and I really dig that. I hope that made some level of sense, it's really hard for me to put into words.