Continental Airlines uses a foreign adapter plug as an input for listening to movies and inflight radio stations on their planes.  On International flights they let you use a pair that they provide for free. On Non-International flights you normally have to pay $5 for them. 

This guide is quite simple, but just by knowing it will same you time and probably money.  I have the Noise Canceling Bose Series III headphones, which comes with the foreign adapter plug, but found it a pain in the butt to keep finding the silly plug and potentially loosing it.  Since no other electronics that I had used the Continental plugs, this seemed silly of me to use. 

SO I tried to use the normal headphone plug in one of the foreign adapter plug, and at first all I heard was sound in one headphone.  Then I twisted the input plug around and pulled it out a little and what do you know.. I could hear in both headphones perfectly. As I stated earlier, this was with the regular Bose headphone input, but I am positive regular headphones do the same thing.  You might have to twist and pull in and out the input plug for a about a min, but YOU CAN DO IT! :)  Once you find the correct place for both sides of the headphones to work, the input plug normally will stay in place.

Hope this helps on your next flight!