EFF decodes information hidden in color print-outs.

What is next

Just so they don't find out about the government eavesdropping device that now comes imbedded in every laser mouse.

And don't get me started on those new blue LEDs that are standard in just about every new electronic device.

And what about all the tracking devices inserted into children's teeth ?

EFF decodes
The article I read appeared to refer only to laser printers. For whatever reason, inkjet units don't seem to have this - yet.