This post won't make any sense to anyone who isn't up-to-date with Lost -- meaning that you watched "Par Avion" last night.
Here are my predictions. We know The Hanso Foundation is a major sponsor of the DHARMA Initiative and heavily involved in life extension research. Eyepatch told us last week that the "Hostiles" (a.k.a., the Others, of which Eyepatch is one) were on the Island long before the DHARMA researchers arrived. So, DHARMA went to the Island to do life extension research but there were people already living there... implying that the Others are actually extremely old themselves, far older than any of them look; some of them are possibly hundreds or thousands of years old.
Eyepatch also told us that the Others can leave the Island whenever they want but that it's hard to come back. He also implied that the Others prefer to be on the Island. Why? Because they start aging when they leave. When the Swan station exploded the Others' sonar beacon was damaged so the ones outside can't get back to the island. This is bad for them since they'll start aging. Perhaps the Swan station was a power source that used the strange magnetic properties of the location to power the beacon and the communication equipment? Or perhaps the explosion damaged the equipment, like some sort of electromagnetic pulse.
Did Desmond avert the end of the world when he destroyed the Swan station? It's implied that his fate was to turn the key, and what would be the point of the key if it wasn't able to prevent the Swan station from ending the world? My theory is that the Swan station or the magnetic location it covered somehow protects/protected the Island from being discovered, and that if it were allowed to overload then outsiders would have been able to find the Island. If the Island grants everyone who visits eternal life, then discovery of the Island could indeed precipitate the end of the world. So now that the Swan station is destroyed, is the Island still impossible to find? It would appear so, since Eyepatch was worried about the sonar beacon, but then he didn't seem to be particularly well-informed about ongoing events. Perhaps the magnetic anomaly was saved from destruction when Desmond turned the key, even though the station itself was destroyed.
So why did Bea Klugh ("be clue"?) want Eyepatch to kill her in last week's episode "Enter 77"? First off, there's a reasonable chance that she didn't die when she was shot. We know the Island can heal those with enough "faith", so it's very possible that Klugh and Eyepatch will both be revived after the main characters have leave their bodies behind. Perhaps Klugh knew that Sayed would torture her if she were captured and that she would reveal critical information, so she thought it was better to take her chances with a bullet. Similarly, why would Eyepatch thank Locke for throwing him into the sonic fence?
If these guys have eternal life, why are they so quick to embrace death? Either because (1) they aren't really dying, (2) they're protecting some idea that's even more important to them than living forever, or (3) they're afraid of some consequence of betraying the Others that's even worse than death. I'm thinking the most likely answer is (1). But Ben has implied (by claiming to be the "good guys") that the answer is actually (2). However, most people aren't eager to sacrifice their lives unless there's an imminent threat to whatever they hold dear, and neither Klugh nor Eyepatch had reason to believe that the Others' goals were in immediate danger.
So my overall theory is this: the Others are the guardians of some sort of fountain of youth. Many of them are extremely old, and despite their youthful appearances they can't have children. They abduct "good" people (and people with "psychic" powers?) to replenish/grow their society, and they reject "bad" people because who wants to live forever with a bunch of rotten apples? The Island is protected from discovery by the outside world by some sort of mystical force, but the force isn't perfect and sometimes people get through, including the Others, the DHARMA Initiative, and the Losties. The Others let castaways live on parts of the Island but do their best to protect their fountain of youth because they know the effect it would have on society if it were found. The Others were eventually forced to destroy the DHARMA Initiative because the DI was getting too close to the truth.
Meanwhile, Penelope Widmore and Widmore Corporation have lost contact with the DHARMA presence on the Island and have even lost the Island itself. Charles Widmore sponsors a yearly sailing race around the world, probably part of an attempt to relocate the Island. When the two Portuguese-speaking men at the listening station detect the explosion of the Swan station they call Penelope and tell her they "found it", obviously meaning the Island. They knew to look for magnetic anomalies, and the explosion was a big one. This means that Widmore Corporation is probably on its way to the Island and will arrive soon. Penelope seems like a good guy, but her father isn't and it's very possible that the Others will turn out to be acting in the best interests of the world at large. The Others' sonar beacon is destroyed, so they won't be able to get reinforcements (if they have any available off the Island) which will force them to team up with the Losties against Widmore.