As several tipsters have pointed out, the recessed iPhone headphone port means that a lot of third party headphones don't work properly: we've tested a few models already (you can see the current list after the break), but maybe you can help us out and let us know which models work / don't work in the comments -- and yes, if you have to press down on the jack to get both channels to play sound, we count that as not working. You'll need to buy a $10 headphone adapter if you want most third party headphones to function correctly, which sounds like a great business plan to us: just break an important device function, and sell the solution for fun and profit. For now, one of the few third party 'phones that do work are the Zune headphones.
Do Work • iPod headphones • Zune headphones (despite the tension in the air, the Zune's headphones do work on the iPhone) • Etymotic ER6i
Don't work • Shure E4c • Ultimate Ears Super.fi 5 Pro • Sony MDR-V150 • Generic wrap-around headphones • Grado SR80