This skimmer was found last year, attached to the front of a Citibank automatic teller machine in California.
This is fairly professional job: Notice how the bulk of the electronics fit into the flap below the card acceptance slot. Also, check out the tiny pinhole camera [on the left, underneath the slot], ostensibly designed to switch on and record the victim’s movements as he or she enters their PIN at the ATM.
Neatorama has previously featured "traps" that physically steal the ATM card, and totally fake ATM machines. To counteract these skimmers, some ATMs "now oscillate the card back and forth as it is motored into the machine, effectively not providing a smooth "swipe" these skimmers need." Other ATMs incorporate GreenSleeves, but criminals have devised fake GreenSleeves!
http://www.krebsonsecurity.com/2010/01/would-you-have-spotted-the-fraud/, via Reddit.
my best advice is to cover your digits with your free hand when entering you pin.