Laser Printers 'secret' encoding

I found this Boing Boing article interesting

Color laser printers add hidden ID number to print-outs
Zach J sez: Colour laser printer manufacturers encode each printout with the printer's serial number so they can trace it back to you if you are counterfitting bills. They can trace it back to you for anything else as well. Oh, and you could of course hack this to give yourself a nice alibi. "Clearly it wasn't MY printer, look at the code!" How long have they been doing this? Why isn't it common knowledge? What other ways to track our lives have been implemented without a big announcement? Link

UPDATE: Anonymous sez: "The answer to the question of how long color laser copiers have been encoding their serial numbers onto their prints is: at least since 1995 or so.