Before tonight's show, I wanted to share my theory on this season: that Jack has already "missed" his opportunity. I believe the entire series now revolves around Jack saving the young Benjamin Linus, preventing him from ever joining the others. Jack refused to save Linus because he knew, positively, that Linus was evil. Had he believed in Locke (the character not the philosopher--its his allegiance to the positive philosophy that's got him in all this trouble), he might have allowed the other to overflow his certainty, to interrupt his self-assurance. He might have given his-self over, face to face. (You didn't think I would get through this without a trace of Levinas, did you?)
This is why his character now seems so, well, lost. Or useless. He has missed his purpose. My guess is that we will eventually see Jack back in time again, in front of a young dying Benjamin Linus. And by that time he will come to believe in Locke, in something beyond positive knowledge, in taking a chance on something other than what he knows.
On a causal side-note: I believe the island designed Sayid to shoot Linus to present Jack with this opportunity. But, this doesn't actually break a linear possibility for time: in the previous past, Linus could have reached the other's any other way. All I am saying is that the island wants to undo Linus' affiliation with the others (perhaps to purify them?). I believe this reading coincides with how the show is revealing much of the island's history from the others's perspective--and, if you were paying attention last week, implying that, if left to their scientific-utopian devices, the Darhma Initiative likely would have undone the island's space-time continuum.
A final note: I think the fact that Sayid's name can be presented as "Say-id," given his compulsive, animalistic, lustful, and emotional nature is probably not coincidental. Then again, perhaps I'm analyzing this a bit too much.