One of the more compelling mysteries raised by I Do (3x6) was Other Tom's cryptic reference to "Jacob's List." Many including myself have speculated that he's the guy with the eye patch whom we glimpsed on the Pearl monitor at the end of Cost of Living (3x5). If so, that would make four Other men with biblically inspired names -- Ethan, Benjamin, and Adam are the others.
Most likely, Patchy has been watching our Losties via the Swan cameras on behalf of the Others for some time. He may even have visited Pearl at some point, and left the cigarette we saw in ? (2x21). Perhaps Jacob was originally charged with watching over Desmond during the latter's quarantine period in Swan. His mission changed when Flight 815 crashed the party.
That's the likely speculation, in my opinion. But if you really want to follow me down the rabbit hole, consider the possibility that Jacob is a code name for someone we know well. I'm talking about one of our most beloved losties -- someone who's been with us literally since the beginning. Who is this mole lurking in their midst? Vincent the dog.
If that seems crazy, keep in mind this might not be the first example of animals serving the Others' interest. The Bear that attacked the party tracking the distress signal in the Pilot, the boar that harassed Sawyer, and the shark with the Dharma logo that attacked the remains of the raft, could all conceivably be examples of such behavior.
Where does that damn dog go?