Thug writes:

The black ink that is used to print the front side of U.S. cash contains a very finely ground black iron oxide powder. The ink is magnetic. This magnetism is used mostly by vending & change machines to distinguish real bills from forgeries/photocopies. [...] 3. Magnetic ink detection. Real bills are printed with magnetic ink, forgeries/photocopies are not.

Magnetic toner for laser printers does exist. Its purpose is to allow people to print their own checks. Peter Baumbach baumbach@atmel.com