I remember reading an article in some journal or other a few years ago about a researcher who carried a spectrum analyser in his tote bag on a number of flights to record cellphone activity (probably with authorization - he was quite an anti-electronic-devices activist).
His results showed a surprising amount of activity on each and every flight. Of course, he interpreted this as being indicative of the magnitude of the problem. But if that is the case, why aren't planes falling out of the sky like flies?