There is some evidence to support the claim, but it becomes a problem with the long-distance cyclists more than anyone else; cycling doesn't promote bone strength because so little weight is actually carried by the bones -- if you STOOD on the pedals the whole time, it'd be a different story.
I began to 'worry' about this in early 2010, while recovering from a cracked ankle (2nd broken bone in just over two years), and started to supplement with calcium/vit. D. No definitive way to tell if it's done any good, but I haven't broken a bone since. Being on your feet a lot will negate any problems cycling may cause, and my job is 'on the feet for 8'.