My PlaneQuiet headphones came with this double pronged adapter that they said I would need.... I've never used it.
All the airplanes I've flown in recently use the 1/8" mini-plug that is standard on most headphones. The only exception was last year when I was on Cathay-Pacific and they use the 1/4" style. A friend and I went looking in Hong Kong for the proper adapter and did find one, so I added that to my bag.