The web page at
http://www.eff.org/issues/printers and several others linked from it
tells the whole story about printers that print their identity with yellow dots. It includes
information about how to decode the dots produced by at least one printer (Xerox Docucolor)
and has a list of many offending printers. There seem to be at least some offending models
from each of the major vendors (HP, Epson, Xerox, Tektronix, Lexmark, Canon, Dell, IBM,
Ricoh, others) Two vendors listed with no known printers with dots are Oki and Samsung.
The Electronic Frontier Foundation (great organization, by the way; consider
joining it!) has had an effort in this area since about 2005, and that web site mentions a couple of
press articles around the last quarter of 2004 which spilled the beans, so this is really not new news.
The dots in the Docucolor are contained in an area that's about an inch by a half inch; there's an
array of 8 x 15 positions, or 120 bits.