I brought it up because I had heard that the original iPODs ROM was extracted using the "click" sound and a microphone. I can't find any reference to it now...

There is also research about using modem lights (even in the background of a room) to figure out what people are doing on their dialup internet connection. Those RX and TX LEDs are actually blinking at your data transmission rate.

Here's the iPod story: http://www.ipodlinux.org/stories/piezo/