We've been discussing this situation here on FR since 11/2004. The left has been busy too. They took George Allen out. Ditto Rick Santorum. I can't think of a prominent conservative who would make a great nominee who is not in the race. Can you?
Sarah Palin? Mark Sanford? Tom McClintock? Herman Cain? J.C. Watts? Janice Rogers Brown? All would also make for some decent possible choices for V.P. on the GOP side.