We've seen it in the movies. A spy or important government agent uses a pre-determined authentication word to not only prove who they are, but also that they are in danger.
I have not seen it implemented directly in any of the big day to day OSes we (the consumer public) use: Windows, Mac, Linux, iOS, Android, Windows Phone, or Blackberry.
I have found an instance of home security systems that have a duress code that will send a signal to the police, but I'm more interested in personal computer or mobile applications.
Does this technique get used in the real world? Are there particular OSes that support a duress code? Could I add this feature to any of my devices through third party software?