Disclaimer: I'm new to lostwiki and have no experience whatsoever in creating pages. Also, I haven't read all of the theories that other fans have come up with. I may very well screw up this "add a page" and my theory may be something tons of other people have thought of. I really want to put this idea out there and see what other fans think. That being said....

Recently it occurred to me that perhaps the island exists in a parallel universe. On one hand you have Earth or Reality or whatever you want to call it, the place that Oceanic flight 815 came from, and then you have a parallel world that is similar to it... the island. This parallel universe is accessible through a portal or wormhole or some other mechanism. When Desmond failed to press the button the magnetic forces contained by The Hatch opened a portal that drew flight 815 into the parallel universe and onto the island. The Dharma Initiative knew about this portal and were able to come to the island through it. They traveled from the "real world" to the island and built their scientific research stations to study the odd phenomena present in this parallel universe. They then built the hatch to contain the forces that would allow unwanted visitors to come to the island. They lied and told people that if they didn't press the button ever so often that "the world would end". What they mean by that is that their private experiment island would be invaded, but they needed to use fear to get the button pushers to be thoroughly committed to pressing the button. The Dharma Initiative weren't the first people on the island though, The Hostiles were possibly there previously. What pitted Dharma against The Hostiles (or the Others) was different ideas on how to control or benefit from the island's powers. Perhaps The Hostiles aimed to hide the island's powers from those who would exploit it (Dharma). This is why they believe they are "the good guys" as they put it from time to time.
Perhaps the parallel universe only consists of the island or a group of islands. That would explain why Desmond was unable to sail away from the island. Maybe the Others used the submarine to travel through the portal back to "the real world". It seems like every aircraft or ship that has come to the island has been destroyed in the process.... except for Desmond's sailboat. Maybe the submarine is the safest way to travel back and forth, something about being submerged affords maximum protection or shielding while traveling.
I realize there are some holes to my theory. How do the food drops keep coming? How is communication via satellite and radio feeds maintained? I also realize that of the dozens and dozens of hours of episodes already aired there may be things that I've forgotten that would ruin my theory. Anyhow, I wanted to put this out there and see what other people think.

Not a theory, but a guess about the future: Whatever the secrets of the island are, it seems they're powerful enough to compel the Others to drastic measures. It seems that Ben Linus learned enough about the island that he was willing to murder his father and at least the majority of Dharma to protect the cause of The Others. Ben and The Others are trying their best to achieve something we're not aware of yet, but at this point (End of the 3rd season) he seems to be failing miserably. Jacob, whatever or whoever he/it is, has already reached out to John Locke and asked for his help. We're lead to believe that all this time Jacob has lead Ben and The Others but now he might be willing to enlist Locke and the survivors of flight 815 to carry out whatever The Others are trying to do. At the end of season three, Ben is badly beaten and tied to a tree, the girls in the underwater hatch are dead, Tom and the war party on the beach are dead, and the remaining Others seem to have lost confidence/trust in him. Meanwhile the survivors of flight 815 are kicking ass. My guess is that in the next season the secrets of the island will be revealed to Jack, Locke and the rest of the survivors and these powerful secrets will convince them to stay on the island and protect it from Naomi the parachute girl and whoever her people are. Ben warns Jack that Naomi and her crew are going to get them all killed... I think that in the next season this will be proved to be true and Jack and the survivors will narrowly escape death and won't be rescued. Of course, the last 4 minutes or so of last night's episode (season three finale) shot all my guesses to hell. It appears that at least Jack, Kate and a few others get rescued. I was starting to like my theory that Jack, Locke and the survivors would become the "new Others", banding with Richard Alpert and the remaining Others to protect the island from Naomi and her people. The scene where Jack and Kate are apparently in a post-rescue future kind of ruins this theory but who knows.

Please comment on these ideas since it's all bugging the hell out of me.