Like nearly everybody else, I've been trying to figure the whole thing out. Who is the man in black who says he wants to kill Jacob? What's preventing him from killing Jacob as he says? What's so special about Locke?
This fan-made speculation video advances at least one theory: the man in the dark suit is the "real face" of the Smoke Monster.
This is a theory I can absolutely buy. The Monster, for the most part, goes after people who "aren't supposed to be there" -- witness its first victim, the pilot of Oceanic 815, who was flying in place of Frank Lapidus.
Another interesting issue: why the Monster chose Locke for his "loophole" mission. Recall that Locke has an interesting history with the Monster -- he was the first of the survivors to see it while out tracking boars, and then it attacked him in the season finale: he refused to run from it, choosing instead to stare it dead in the eyes, and it floated away.
"I believe I was being tested."
Later, he described what he saw to be "a bright light" to Mr. Eko, who said, "That is not what I saw." Later, Locke goes to rescue Eko from a polar bear, only to eventually lose him to the Monster (Eko's dying words: "You're next").
The next time he mentions it is at the beginning of Season 4, while holding Ben at gunpoint -- Ben offered him information, and strangely, the first question of Locke's mouth was, "What is the Monster?"
One other theory I enjoyed, courtesy this video on youtube: the conflict between Jacob & his nemesis is a more cosmic version of the conflict between Jack & Locke (which, as we've argued before, is a microcosm of the conflict between science vs. faith, and predestination vs. freewill). Tell me if there's any parallel between what's going on in this video, and the one we viewed at the beginning of the post.
As always, post your own theories and thoughts in the commentary section. And remember, the finale is right around the corner.