Anyone wondering what Lost is? It is now almost known. It is basically a ongoing competition between two deities both trying to prove their point. Jacob believes Man is inherently good. "Steve" believes Man is evil. THey both test man to prove that they are correct. Jacob isn't dead, it's just a part of the test. The test was to finally demonstrate that Ben was indeed evil. Jacob even inferred that Ben was selfish and that was why he never sought an audience with him. Ben failed this test. Ben could not kill Jacob if he had some special weapon let alone a knife.Jacob or Steve can not die. But they can appear to die to make things interesting. I think the killing scene of Jacob is setting the next phase of Jack's character. As "Steve" is possessing the form of Locke, Jacob will come to possess the form of Jack.I believe Jacob and Steve are bound by the same rules so Jacob will be back as Steve is just in the form ! of Jack or a good twist could be Sawyer. I doubt Sawyer will be the proxy though due to the earlier themes of science vs faith with Locke and Jack being representative of each. Maybe there's some device within the guitar case Jacob gave Hurley to facillitate this process. Steve and Jacob as Locke and Jack will lead their forces to finally do war. I am not exactly sure how this will end but most likely as in life their will be no definitive ending just realization that Jacob and Steve are both right and their war will come to a draw with neither losing. It is somewhat like the temptations of the Garden of Eden in a way with a lot more elements mixed in.