Recent studies, discussed in a NYTimes blog have found that cycling is a risk factor for low bone density or osteopenia. This finding is actually not unexpected. Bone mass is in part determined by the amount of load / force applied to the bone. Thus, as noted in the blog runners, weight lifters and triathletes have higher bone mass than cyclists (keep in mind that triathletes do a significant amount of running too).
These findings stress the importance of weight bearing exercise and especially for females. Swimming and cycling while both great exercises do not put adequate stress on bone to stimulate bone deposition thus predisposing the swimmer and cyclist to osteopenia and osteoporosis.