In the UK it's covered by the Wireless and Telegraphy Act 1949 which basically says you are not allowed to listen to transmissions to which you were not the intended recipient. In practice people use scanners for listening to airband, fire, police, ambulance, taxis etc. with little problem *but* if you are silly, following the police etc.) you would be in big trouble. The main method of establishing guilt is if the freq. is programmed into the scanner, otherwise you are ok. Some police forces use encrypted radios now, Bournemouth is definately one of them but it is a pretty expensive thing to do and it will be a few years before they are all converted.

Gareth