[pg314]

AFAIK there is no evidence that running is bad for the knees. There is not enough research to draw firm conclusions, but a Stanford study (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556152/) didn't find a higher incidence of osteoarthritis in runners (20%) vs non-runners (32%) after 18 years of running. There is selection bias in this study since the runners were self-selected.

The book 'Which comes first, cardio or weights' from A. Hutchinson offers one hypothesis in which running might protect the knees: by limiting weight gain with advancing age. Less weight means less stress on the joints.