From the scientists:

Cycling is low impact, which is great to keep joints healthy, but bad for building up bone mass. They seem to be saying that some impact exercise is helpful for healthy bones... Falling a lot doesn't count!

http://peakperformance.runnersworld.com/2009/01/jan-12-the-truth-about-exercise-and-weight-loss-fit-people-save-1500yr-on-medical-expenses-zma-doesnt-boost-testosterone-cyclists-have-low-bone-mineral-density.html