I came out of retirement just to post in this thread.
KarmaSutra is right. I was just about to post a lengthy thing about time loops but I edited most of it down because it's been covered.
Many of the Losties (if not all) are already stuck in time loops. This also explains the ability of dead people (Charlie, Jack's father, Eko's brother), living people not on the island (Walt), and people from the 'mainland' (Locke's "father") to suddenly appear there. The island seems to be some sort of portal to different dimensions in time, and apparently many of the characters are stuck in a loop (most notably Desmond and Faraday).
Evidence is also pointing to Hurley and Jack being stuck in a loop. In the "flash forwards", why does Hurley hang out at the mental hospital even though he's not clinically insane? Because he's trying to recreate the moment when he jumped back in time, kinda like when Desmond tried to do that in vain in the sick bay. When Charlie slapped him in the face, it WAS real. Even though in one dimension of time, Charlie was dead, but in another reality, he was alive and was able to visit Hurley.
In his flash forward, Jack talks about how every time one of his flights goes through turbulence, he prays that it will crash. He's trying to recreate that time travel, or revisit the other dimension, if you will.
Alpert seems to be the only one who can travel through time at his will. But I'm wondering if Ben Linus can do the same thing. So far, it seems like they're the only two characters who can come and go from the island as they please.
Thoughts?