In most of the cases about which I've heard, the plaques where simply moved across the street, or otherwise off of governmental property. That reminded me of George Carlin's take on the situation:
"The real reason that we can't have the Ten Commandments in a Courthouse? You cannot post "Thou Shalt Not Steal," "Thou Shalt Not Commit Adultery" and "Thou Shall Not Lie" in a building full of lawyers, judges and politicians...It creates a hostile work environment."
