With the operative word being "may".
Link to October 27 textually.org post, "Cell Phone May Reduce Bone Density in Hips".
Link to October 27, 2009, WebMD Health News article, "Cell Phones on Hip May Weaken Bone. Study Suggests Link Between Bone Weakness and Wearing a Cell Phone on the Hip".
Excerpt: Using an X-ray technique used in the diagnosis and monitoring of patients with osteoporosis, researchers from Turkey's Suleyman Demireli University measured pelvic bone density in 150 men who regularly carried their cell phones attached to their belts.
The men carried their phones for an average of 15 hours each day; they had used cell phones for an average of six years.
The researchers found that bone mineral density was slightly less on the side of the pelvis where the mobile phones were carried than on the side that was not in contact with the phones.
The difference was not statistically significant and fell far short of approaching bone density reductions seen in people with osteoporosis.
BONUS ROUND (August 26, 2012)
For those of you for whom the operating word is "meh", these all-leather cell phone pouches from Australia -- have Velcro closures and slip upright on any belt -- are certainly an eye-catching accessory.