While Lost is certainly liberal with killing off main characters (especially when they fall in love on the island), that doesn’t always mean those characters disappear entirely from the show. Lost is the king of flashback scenes, creepy visions on the island, and now as of the end of Season 3, flash forwards too.
So all this creepiness and time-jumping brings “back” characters like the dear Boone who was among the first to die on the island, and now Hurley‘s love, Libby, who was taken down in the bout of Michael‘s insane attempts to get off the island.