In addition to your flight ID every mode-S transponder also emits an 8-position octal (24 bits) ICAO code. This code is unique to the airframe and should not be changed by the pilot at all.
These ICAO codes were assigned in bulk by the authorities. You can see them, for instance, in G-INFO. And I have not checked but I would assume the N-reg check on the FAA website would show them as well.
Last edited by BackPacker; 26th August 2012 at 21:18.