I've read about 5 articles in the past couple of days about the "Mosquito" ringtone that has been going around in the US and UK.
Here's a few links: BBC, Washington Post, CBS News.
Anyway, the story goes like this:
A short time ago, a Welsh security company decided that a good way to stop teenagers congregating in certain areas would be to broadcast a really annoying sound that could only be heard by youngsters and not adults. The idea relies on the general premise that your hearing degrades as you age. The other articles have better descriptions and I'm too lazy to put my own up... There's an image on this post that gives you the idea anyway.
Anyway, some enterprising young teenager decided to turn the technology against the adults, and recorded the sound to use as a ringtone for his phone. That way, he could use it in class, and the teachers wouldn't be able to hear it. One of the posts I read suggested that maybe he could have just put the phone on silent, but I've never been a fan of occam's razor (boring), and this solution is much more crafty.
It's worth looking at the blog post by a guy named Ochen Kaylan - he gives samples of a whole lot of frequencies so you can see when yours cuts out (the last one I can hear is 15,000Hz - I'm going deaf...)
Oh yeah, and this guy has a download of the mp3 if you want it for your ringtone.
Damo "Visa" Brady