Apparently, kids are using a ringtone that adults can't hear [1] to send messages to each other during school (link via Shadesong [2]). Well, we have some kids around here, so I thought I'd try it [3].
I couldn't hear it. Neither could Wlofie. The Kids—they could hear it plainly.
It does remind me of one story from high school. One of my science teachers was color blind, and some enterprising students (and no, it wasn't me) used that fact to cheat—they would write the answers in red on a red folder that was left out in plain sight. The poor teacher wouldn't notice, but the students could (darn! Now why didn't I think of that?).
[1] http://scienceblogs.com/cognitivedaily/2006/06/can_you_hear_this.php
[2] http://shadesong.livejournal.com/2897036.html
[3] http://scienceblogs.com/cognitivedaily/upload/2006/06/ring.mp3