An article in the July 1, New York Times raises just that question. The article cites several scientific studies published over the past year and a half. These studies all show an alarming loss of bone density among cyclists.
These studies will likely spawn further research into the mechanism by which cyclists lose bone density, but for now the reasons are unknown. Some researchers speculate that cyclists lose too much calcium through sweat.
The good news is that, unless you are a racer, you don’t have to be very concerned about these findings. The studies all involved racers. At present there is no indication that recreational cyclists need to worry about bone density loss.