The New York Times:Technology:Circuits:Block That Ringtone!
by Sam Lubell 4/8/04
..."companies and researchers developing or already selling devices that render cellphones inoperable in certain locations. Methods include jammers that interfere with cellphone frequencies, routing systems that mute phones' ringers in specific places, sensors that detect active cellphones and building materials that block cellphone waves."
Sounds like something to build into new library design. Well, there are some emergencies when you need a cell phone on. Wouldn't it be nice if people just silenced their phones in libraries, movies, etc?