Lots of people have speculated about ABC’s Lost being about time travel. Especially since the “Flash Before Your Eyes” episode in which Desmond’s flashback has him back in his past but remembering the island; then he’s talking with some Creepy Old Lady (an actor from the film The Others) who talks of fate; then as soon as he’s whacked on the head in a bad he’s back on the island turning the key before the Hatch exploded; then he’s suddenly naked in the brush (reminiscent of time travelers in Terminator flicks). On the island, now he knows the future — just as he did when his consciousness was temporarily back in his past self.  Loop-de-loops in space time. 

Now let me see if I can add some wrinkles:
(1) Naomi, the parachutist supposedly from a rescue ship, tells Hurley that everyone on Oceanic Flight 815 — the castaway’s plane — died. Maybe they did AND they didn’t — they’re still alive because they were sent back in time. Yes, they’re living in the past.
(2) If they’re in a past time-frame, that might explain why Locke’s not paralyzed and Rose doesn’t have cancer any more. Their new time is when they were well (even as they inhabit their older bodies).
(3) Maybe that’s why women who get pregnant on the island always die during their pregnancies. The baby’s soul belongs to one time frame and the mother’s to another, and as birth approaches, the universe objects.
(4) Maybe that’s why death seems slow to hit its mark. The Russian got zapped and showed up fit as a one-eyed fiddle later, explaining “On this island, the wounds are a bit different.” Then he fixes up Naomi, who quickly recovers. Locke looked mortally wounded, too, but soon he’s on his feet, healthy enough to knife Naomi down. (As Creepy Old Lady said to Desmond, “The univerise, unfortunately, has a way of course-correcting.” Naomi’s turn returned. … as do Charlie’s and the Russian’s.)
(5) That’s why Jack’s father’s coffin was empty. And why Jack’s father seems to be alive during the flash-forward Season 2 finale. Jack’s looped back to a time before his father died. 
(6) That’s why Jack has to go back. He knows he’s messed up the world’s time line somehow, and the course has to be corrected. Before some villians / scientists / military types try to control the island’s powers and Earth’s destiny … with dire consequences. I suspect Desmond may have left and returned himself (remember his meeting Jack in L.A. and saying “See you in another life”?) … he didn’t just “see” Clair drowning and Charlie dying … he’d lived through it before … like Ground Hog Day.
(7) No wonder Ben is raving mad. You would be, too, if you were born on an island out of time, knowing no one should ever find it and exploit mysterious magnetic powers.

Theory doesn’t explain everything. Not even coupled with a previous theory: “The Scared, Intelligent Island.” 

Somehow it needs explaining how Locke’s father suddenly appeared.  Ben said the island is a “magic box” that made wished-for things materialize. This isn’t just a matter of time, it’s a matter of matter and space. The whole space-time-continuum thing.

Does the magnetic island have a host of mysterious natural powers?

Or does it basically have one: the power to naturally amplify the psychic powers of time-trapped minds? Healing, visions, clairvoyance, fantasy, even telekinetic teleportation? 

Or, coupled with the Fly / Forbidden Planet Theory, does a very human consciousness pervade the island, work with the souls aboard, and make miracles and nightmares happen?   

Anybody out there ready to put this all together? “Help me!”  

Or maybe they do … the magnetic island has many mysteries … which we’ll never learn.