When we brought Ebony home he was naked in areas of his body. He had licked his hair off his belly and his inner hind legs. I feel it was due to the stress he was in at being left at the shelter and being their for 4 months.
I am happy to report that his hair is growing back. The vet will be pleased. She noticed it and told me to keep and eye on him.
Here is an article on this subject which can be a serious medical condition for a cat: http://vetmedicine.about.com/od/diseasesandconditions/f/FAQ_hairloss.htm
You can Google the topic and get more hits try: cat hair loss or pet hair loss or baldness in cats