It's been a question for some time as to whether Jack will have to make the ultimate sacrifice by the end of the show. I think he will.
Why?
Well lets start off with his relationship with Kate. Kate the heroine of the show, and Jack the hero. In any story you've ever heard of, it's very very rare that the hero doesn't get the heroine in the end.
Unless of course, the hero tragically dies. Does that mean Jack will die? No, but,,,
We've already seen the Jack & Kate relationship. We saw them meat, flirt, kiss, say I love you, fall out of love, try again and fail misserably. It's not impossible that the show would reunite them as a couple, but from a storyline perspective, it's all already been done. A more fitting conslusion to their story at this point would be for them to decide that, yes they do want to be together, only for Jack to die before it happens.
Also we need to consider the importance of Locke. He died, or was led to his death, in some sort of sacrafice on behalf of some cause or person we don't yet understand. But Locke, while he left the island willingly with the knowledge that he would die, did not in fact die willingly.
His death was meaningless, at least in the traditional "hero gives his life for the greater good" sense. So where is our classic hero death in the show? Daniel,,,, no he died in much the same way Locke did, confused and without knowledge of what was really going on. Juliet?
Oh, yes she did die heroically. And before her Charlie was the ultimate hero. Charlie was a great death and his death elevated him to a higher category of hero. Before that he was less, and so you'll see that there has been an increase in the calibre of hero dying from Charlie up to Juliet. What we need to see is the next step up, from Juliet's level, to the ultimate level and the ultimate hero that is Jack.
But going back to Locke. It would seem that if you have such a strong character in Locke, arguably the hero himself of the series, and that his death was pitiful, that you need the primary hero of the series to have a death of his own that is made in true sacrafise for the ultimate of great goods. It only makes sense that if Locke was used in death, which lets us down as an audience, that Jack will be the opposite, and lift us up.
So there are two logical storyline based reasons to believe that Jack will die in the end.
But are there others? Well there is a parrallel between how Charles & Eloise led their own son Daniel to death. Clearly Christian has shown eerie signs of having some sort of knowlege about the island. Was he the Man in Black all along? Perhaps, but if not, and I think there is a strong reason to believe that Christian is Christian in some way, that you have to wonder why he's guided Jack the way he has, and did he die in the real world on purpose in order to bring Jack to the island? Was Christian's body what led 815 to the island? There has to be something to that.