Unobserved people paid almost three times as much to the "honesty box" when watched by a photocopied face than in the absence of the face. See also What the Bagel Man Saw by Freakonomists Stephen J. Dubner and Steven Levitt. I wonder if the smiley face on milk cartons deters people from drinking straight from the carton?
Update: Spiegel Online has a story with a graph depicting the results of different sets of eyes...sexy eyes did the worst while serious eyes looking straight ahead performed the best. (thx, roland)