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.
"The Canon CLC 700 was introduced then and didn't come with the any discernible anti-counterfeiting features. The previous generations of the Canon CLC lines detected that they were being used to copy currency and put a blackish green cast over the output. The problem with the old system was that the detection was too sensitive and would trip on non-currency items and not detect the planned new currency designs.
"After a bit of looking (and some pointed questioning on behalf of our security-conscious customers) we found about the yellow-dot encoding, and sure enough, it's been on every color machine since.
"The reason you probably haven't heard much about it is because it (AFAIK) is only used by the Secret Service to trace counterfeit documents back to their source machines, and the Secret Service doesn't like to talk too much about means and methods."