Any output device is an output device. A VGA interface. An HDMI interface. A Scroll lock keyboard light. A hard drive interface. A speaker. All you need to do is send the signal down one wire and you could tap into that wire and copy all the data to another system.
Copying files from one folder into another could do the job.
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.
