What if Specter dies?

I know this is a ghoulish topic, but the latest Fineman piece in Newsweek says that Sen Arlen Specter (R-PA) is battling "Stage IV Hodgkin's" disease. Obviously, that's pretty serious. What if he doesn't make it? What if there are two Senate seats on the ballot in PA in 2006, Santorum's six year term and the remaining four years of Specter's term?

Who would Rendell appoint to the vacancy? Would he run himself? Which seat would Casey run for?