was thinking that the people who got sick on the island might have been quarantined to one side of the island-the side that michael and jin and sawyer land on. that is why the security system is near the black rock. it is to keep them on that side of the island or to keep others away.
also, the beeping over the speaker when desmond and locke were in the hatch could be the beeping radar of the boat returning to shore and the "numbers" he had locke type in were the coordinates to lead the boat back to the island.
one more thing, michael has powers too as he made the raft break and sawyer lose his bullets. i think this confirms my theory about the men and dads. walt had to be raised without michael as claires baby had to be raised only by her. rousseaus baby was fatherless too as the dad got "sick" and she killed him. all three were males, ad claire visited a psychic.

Those are some really interesting theories you have there! I agree that the island itself is quarantined, and thats why the quarantine sign is on the inside of the hatch door, as opposed to being on the outside. Also, that could explain what Desmond is shooting into his arm; some kind of vaccine against the so-called island sickness. (Same one he was asking Locke about; whether people had caught it and died.) Can't wait until they reveal something about this!