The Athlon heat problem was pretty well described in an article at Tom's Hardware. Athlon doesn't disipate any more heat than P4 but due to lack of internal thermal management it will self destruct without proper cooling. It can take the MB with it as it frys. (such as a fan failing, power connector coming loose, bad heat sink mount, etc).
On the otherhand, P4 automatically throttles down gracefully if it detects an overheat situation.