Great story I came across on NPR.
(You can listen to the full story from a link there using realplayer)

The war between teens and authority figures has a new -- or old -- front: ears. British shopkeepers tired of teenage loiterers have turned to the Mosquito teen repellent, which emits a high-pitch frequency that most teenagers can hear -- but not most adults.

But now teens have struck back against the Mosquito: They are using the same sound to communicate without adults' knowledge.

At issue is a text-message ringtone that emits the same pitch as the Mosquito. Using it, students can learn about a new message while they're in class -- where they're not supposed to be using their cellphones. Most of their teachers can't hear the alert.