I think I recall reading about this one too. There are all kinds of ways processors and devices leak information. If you have the time and access you can correlate a lot of things to specific hardware and software operations.

I remember hearing something about a typewriter bug that is tuned to determine the key pressed by the particular acoustic signature of each key press. These kinds of outside-the-box solutions really interest me.

EMI and crosstalk is another area where information can leak and there are already snooping devices that work using that.