Originally Posted by Stewie Mac

The reason that those nice headsets have two jacks is to make them less attractive to steal because they only work properly on the plane..

One of the jacks is the 'usual' stereo jack, which you can plug into your laptop. etc. The other jack gives power to the noise-cancelling system in the headset - so if you do plug one jack into your laptop, you won't get the noise cancelling effect. Which the airline hopes makes you much less likely to take them with you...