Sayid will kill Jack
I sure hope it's this one!
It won't be. Matthew Fox has already mentioned (I believe back during Season 4) when he was on Jimmy Kimmel that he is one of the few people that the producers have told the whole storyline to and how everything will end up and he's part of it.
Like it or not the show is about Jack. He is the main character and the main focus of the series. He will live until the end of the show (I mean, he could die in the last episode of the show saving everyone else, but he will make it to the end.)
It's like any show with numerous characters, everyone may have their favorite, but there is always one main character you can be sure will make it to the end. It was Picard in TNG, Sisko in DS9, Jack Bauer in 24, Dale Cooper in Twin Peaks, Mulder in X-Files, Peter Paterlli in Heroes, and Jack Shepard in Lost.
if Sayid does "kill" Jack it will be as part of a ploy to fake his death for some reason, or it will be an "alternate timeline" version of Jack that won't effect the current Jack on the show.