My girlfriend was just pulled over by a motorcycle cop that was hiding behind a sign. I know its not entrapment, but I know in some states/counties its illegal for cops to hide, they need to be clearly visible.
I have to agree with Mr. Buckley. While it appears to be completly unfair it does not appear to be against any statute or law. I am not aware of any law that states the police officers have to be visible in order to give you a ticket.