Attempting to ban devices is futile at best and likely psychologically counterproductive.

Anyone with evil intent can easily smuggle devices in. However the fact that such rules would affect people's legitimate and (when properly used) harmless products like ipods, phones, etc. will undoubtedly build a wall of resentment, and perhaps a culture of rule violation (everyone knows everyone else is doing it.. and everyone feels it's justified).

There is no foolproof answer, but the key is in the traditional means of HR and management policies (prevention of embezzlement is a similar problem, and there is much experience at handling it) and with securing access to data (including locked USB ports on many machines).

People are not machines. They do not work well when locked down. They are not loyal when locked down. Where people are treated as responsible adults (including incouraged to take personal responsibility to help protect the company's data) you have much more success in spotting the troublesome individuals.