Of course, the airline gives away those cheap headphones with the dual prongs, but they're not useful for anything except jacking into the plane's own audio system. Unless you hack it, that is. I'm not saying you'll want to do this frequently, but if you have no better options, it's actually pretty easy to rewire the airline headphones so you can plug them into your iPhone.
Lifehacker lays out all the headphone surgery in step-by-step fashion. You don't need any tools -- just a bit of chewing gum.
You'll need to remove the foam from one of the ears, pop off the plastic covering, and rewire it so you can get sound in both ears even though you're only plugged into one of the jacks. It won't be stereo, but then again, there's nothing inherently wrong with mono. Just ask all the audiophiles that bought the Beatles mono remastered box set last year.
Be sure to check out the Lifehacker story for complete details, and you'll never get stuck on a plane with nothing to listen to again.