In some of the locations that I work we have a 'switch off' mobile phones rule. As it interferes with the radio and othre equipment nearby. Not new news, but we recently investigated putting in a loop around the doorway to detect the presence of mobile phones. We apprached Vodaphone, Cellnet and co and talked through our problem with their tech people. The answer we got was that each type of phone (Orange, 1-2-1, Voda etc) all use slightly different frequencies and that every 20 minutes or so they perform a call home, where they transmit their presence and detect the nearest cell. During the 20 minute gap unless they are called or transmit they are silent and are in listen mode.
This is what makes them difficult to trace - different frequecies and a silent period.
We also investigated the possibility of detecting the presence of circuits, this is the best option, but price put us off.
As somebody said earlier the 'best' method is to listen for that familiar interference on the r/t during pre-flight and taxi, then if you hear it slam on the brakes and give everybody the 3rd degree.