I wore out the cartilage in my knee. Was running bone on bone. The quality of my runs degraded significantly over time and it became very painful, even when sleeping. I tried everything, knee brace, chiropractor, drugs but nothing worked. Developed arthritis after surgery for a medial meniscus tear in 1990. Didn't help crashing my bicycle at high speed and knocking the heck out of my knee, too. End result was a knee replacement in December of 2008. My doctor (ortho) and most importantly my physical therapist,said no running. The repeated impact will eventually loosen the glue the helps anchor the device and damage the bone where the the prosthetic fits. They advised that if I ran on it, I would eventually need knee revision surgery which I do not want to have to experience as the original surgery and rehab was quite painful. On a side note, the therapists told me that because I had been a runner, the muscles surrounding the knee were in great shape and very tight, which resulted in a lot of pain as I worked thru the therapy. The people who aren't in good shape have looser muscles and it's easier(less painful) for them to start flexing the knee joint.
Can't run now so I ride a bike, work the eliptical and walk. But my knee feels so much better that I constantly think about running on it.
Hope someone else who has had a knee replacment has been able to resume running and hasn't experienced any problems. Good luck.