There is a new anthology out called What Have You Changed Your Mind About? (Harper Perennial), in which 150 “big thinkers” describe what they now think they were wrong about earlier in their lives. Much of this has to do with technology and education. Some of the more interesting flip-flops come from Ray Kurzweil, who no longer believes in extraterrestrials, and AI researcher Roger C. Schank who no longer thinks we'll have thinking machines as smart as we are in his lifetime.
You can find more opinion flip-flops here
Interestingly, no one I noticed mentioned changing their mind about George W. Bush.