From TV Guide:
Exclusive! Lost Resurrects Libby!
Cynthia Watros Returning to Lost for Multiple Episodes
September 20, 2007 (New York, NY) – Technically, the ABC hit isn't bringing Cynthia Watros' short-lived character back from the dead. But Lost executive producer Carlton Cuse confirms to TVGuide.com’s Michael Ausiello exclusively that the actress will be playing Hurley's sweetheart again this season for multiple episodes, presumably in more of Desmond's flashbacks.
"She'll be in enough of the show for us to fill in the missing pieces of her story," says Cuse. "We could not be more pleased. Cynthia is a smart and engaging actor, and [executive producer] Damon [Lindelof] and I have some very cool parts of her story left to tell."