This is an email I received and am passing on.
Subject: Good to know....
Check with your bank. Hope it never happens.
PIN NUMBER REVERSAL (GOOD TO KNOW)
If you should ever be forced by a robber to withdraw money from an ATM
machine, you can notify the police by entering your Pin # in reverse.
For example if your pin number is 1234 then you would put in 4321.
The ATM recognizes that your pin number is backwards from the ATM card
you placed in the machine. The machine will still give you the money you
requested, but unknown to the robber. The police will be immediately
dispatched to help you.
This information was recently broadcasted on CTV and it states that it
is seldom used because people don't know it exists.
I checked with my Renfrew Bank of Nova Scotia to see if this was correct
and staff said yes this information is correct.
Please pass this along to everyone possible
I believe this is actually a myth. I wouldn't advise trying it.
"Since it is extremely unlikely to work, the "advice" in this message could actually be dangerous. Forcing a victim to withdraw money from an ATM is a high-risk, violent crime. If a victim enters a reverse pin at an ATM that does not have the safety pin system installed, he or she will receive an error message and no money will be dispensed. This delay could antagonize the criminal and increase the risk of violent retaliation."
http://www.hoax-slayer.com/reverse-pin-ATM.shtml
http://urbanlegends.about.com/library/bl_reverse_pin.htm