In the event McCain croaked, Mitty Mitty Bang Bang would get his spot.
He'd probably have Palin foisted onto him as his VP, but since she was nominated and not chosen through a primary process, it would be anathema to stick her on the top of the ticket. I'm not sure there's a precedent, since in the old days before the party system was firmly established, the runner up on election day was the VP.
I supposed they could reconvene the delegation and ask for a do-over with Palin as a scribbled write-in, but I believe the party rules say that if McCain dies before the election the runner up in delegates gets the ticket.
Same thing, if Obama has a freak heart attack before Nov 4th, then we get HRC as the Dem candidate.
On the other hand, AFTER the election, if McCain wins but dies before inauguration, or if Obama wins but dies before inauguration, per the Constitution the VP is inaugurated as if they were elected.
So it's in the evangelicals best interest to keep McCain alive until Nov 4th, then mix up his meds on Nov 5th, if they want a Palin prez.