Powell-Rice in 08 baby!
That said, I think Bush has done one thing extremely well, and that's stick to his guns. While Kerry has waffled and taken both sides of every issue, Bush has held firm and stayed with his vision 100%. I don't see him dumping his VP for a bump in the polls.
However, if Cheney quits for health reasons, I think Guiliani would be an acceptable alternative, and it positions him for the 08 battle of the century against the crazy woman who wants to triple our healthcare costs and stifle all future medical research and development.