I'm a high mileage cyclist (500,000 miles). I drove my heel bone through the tibia/fibula a decade ago. When the X-ray came up, the orthopedic surgeon, one of the team physicians for the Sacramento Kings, called the entire staff in to marvel at the densest bones he had ever seen. If I worry at all about bone density, it is to be concerned that I am dragging excess weight up every hill.
My take on it is this: bones will grow denser in response to damage. Damage is inflicted by stress. Most folks focus on muscle output or weight bearing as primary stress, but I think that may be mistaken. The stress that is relevant for weight-bearing activities for bone density is the micro-fracture causing slamming of the feet into the ground, such as is experienced in basketball. I think I get a similar level of stress by riding full out on crappy roads. The bike bounces, as do my lower legs.
It would be interesting to compare track cyclists' bone density to that of off-road racers, particularly if one can find off-roaders who have good form (less upper body use and more shock absorption from the legs). Maybe that can't be done anymore since the off-road folks are all on suspension bikes.