This was emailed to me. I don't know how accurate it is:

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 FOX TV and it states that it is seldom
used because people don't know it exists.  
Please pass this along to everyone possible.  

I think that's an urban legend.  I think if you go try it, you will find that your pin doesen't work.  Let us know.  

Nah, I think that I will pass on this one. I'm afraid the ATM will suck in my card and then I'll never get it back. I was hoping someone else would try it and report back.

All it will do is advise you that the PIN you entered is invalid.  I just tried it.  Google PIN # reversal...

Thanks! Now we have actual proof that it is not true. I wonder how many people actually try it?
I was visiting some family tonight and was telling them about this. They also called it an urban legend. But they said that they heard that you can enter 911. Did you hear anything about that? Or were they just pulling my leg?

Never heard that one

I checked this out too.  Snopes.Com (a good place for checking any of these "facts") says it's false.