The September edition of the Fed Files column is on voice encryption and security in federal radios. During research, I found this page, from the Department of Management at the US Department of Agriculture:
http://www.dm.usda.gov/ocpm/Security%20Guide/V2comint/Intro.htm#Intercepting
It shows that all federal agencies, from the FBI to the USDA, are at least trying to let their people know about the possibilities of intercepted communications. The page does not specifically mention radio systems, but I suspect most USDA employees don't' deal with radios on a day-to-day basis.