There is an article in the Baltimore Edition of CBS Local about keeping cell phones out of prisons. According to the story, the Maryland Department of Public Safety will attempt to:
block covert inmate calls by creating a radio frequency umbrella around prisons.
Supposedly, this will allow legitimate calls in and out, but block calls from unauthorized cell phones. It sounds pretty good in theory, and will be nice if it works, but the second it does, the bad guys are just going to hire some geek from Harvard, or Cal Tech, to break the system.