This crazy theory is based on Michael Emerson's statement that we've already seen Jacob in a non-speaking role. This isn't an attempt to say who Jacob is, it's more wild than that.
In the Biblical story of Jacob, God said to him "I will give you and your descendants the land on which you are lying. Your descendants will be like the dust of the earth, and you will spread out to the west and to the east, to the north and to the south. All peoples on earth will be blessed through you and your offspring. I am with you and will watch over you wherever you go, and I will bring you back to this land."
Could Jacob have been out in the world, doing his Jacob stuff, and knowing that God would "bring him back to the land", been returning to the Island by way of flight 815? Whatever name he was using would be on the manifest, we'd see him on the beach, and when the Losties flashed to the 70's, he'd have flashed with them and been able to find, and enter, the cabin.
He could have been on the Black Rock and even the expeditions that led the U.S. Army and Dharma to find the Island.
Can this theory explain why we can't we see or hear Jacob, why he's a ghostlike figure?
When Ajira 316 comes to the Island, the Losties who should have been on the Island during the flashes didn't crash on the Island, they flashed to the time frame that they should have been in.
Assuming that Jacob was with the survivors of 815 and did flash to the 70's, with the Losties, and take residence in the cabin, he should have bounced through time with them when they eventually left the 70's. Whether the next jump is to the time of the Statue, the Black Rock, back to 2008, or to each of them in turn doesn't really matter.
What if, during the 3 years that the Losties are in the 70's, Jacob was trapped by Richard and that the ashes around the cabin somehow kept him from flashing to the next time frame? Could this have caused his current limbo-like state? Like, in 2004, when John first comes to the cabin, Jacob should be in 2008. He's trapped in between and has to put out a huge amount of effort to slightly pull John into limbo for him to vaguely hear and see Jacob.