The FCC was kind enough to answer it yesterday.
As of Sept. 30, 2006
There were 13,793 radio stations on the air in the U.S.
4,751 were AM stations
6,252 were commercial FM stations
2,769 of them operated under educational licenses or noncommercial licenses.
Image stolen from this truly brilliant Advertising blog. Their radio articles are very solid, probably a radio man. http://brandingadvice.typepad.com/my_weblog/advertising/index.html