From what I've read, the death is supposed to happen on the season finale - big surprise there. Locke is going to be indirectly responsible for the death, because he didn't tell the others about something he knew about (Maybe the hatch? Or about the monster-thing he saw?). If what we heard before is true, about the person dying as a result of a misplaced/betrayed trust, that could point to Boone cause we all know how he's Locke's faithful lap-dog right now. I'd be fine with Boone dying, personally.