More stuff I've theorized about Jacob (relating to him being the island's spirit), *no spoilers from episodes that have not been aired*:

Remember the black powder that Locke saw before they arrived at the house? I thought it was gunpowder at first, but perhaps it is actually the black smoke monster? They could be iron filaments that are being controlled by the electro-magnetic forces on the island (which in turn, may be being controlled by Jacob).

Richard, the guy who isn't aging, is a hallucination created by Jacob. Richard never seems to talk to more than one person at a time, so he may be a hallucination, just like Kate's horse, Ben's mother, Jack's dad, and so on.

Something else interesting, when Ben is talking to Jacob, Locke cannot see him. However, when Locke hears "help me," and turns around, he sees Jacob sitting in the chair for a split second.

Jacob also could be the cause of the whispers in the forest. The whispers always lead people, mostly to their death's, or Ben across the fence to the hallucination of his mother.

From Lostpedia.com:
In Jewish history, the tribe of Benjamin's women and children were all killed, and the remaining members of the tribe could not longer proliferate. The historical solution they came up with was to kill the men from another sub-tribe and take their women and children. Women are unable to carry to term on the island, and the Others have been taking women and children from Flight 815.
Also, the name of the previous episode was Man Behind the Curtain. Possible reference to Wizard of Oz? If so, perhaps Jacob was just another one of Ben's mind games?