Hint to criminals using computers–don’t.  The following is just the tip of the iceburg.  From Bruce Schneier:

Many color laser printers embed secret information in every page they print, basically to identify you by. Here, the EFF has cracked the code of the Xerox DocuColor series of printers.

Update: For those of you who didn’t really get what I was hinting on the first pass here it is spelled out for you:

Schoen said that the existence of the encoded information could be a threat to people who live in repressive governments or those who have a legitimate need for privacy. It reminds him, he said, of a program the Soviet Union once had in place to record sample typewriter printouts in hopes of tracking the origins of underground, self-published literature.