This is exactly what I did with my FM-tuner and headphones. Don't be afraid, it's just solid metal surrounded by rubber, and you only need to trim a 1/4 inch. Much better solution than spending money on that ugly 3 inch adaptor.
I have the Rebel iPhone Headphone Adapter from BluJuicz. It works great with my Bose in-ear headphones (yes, with the black and white cord). Best of all worldsThe price is reasonable too.
http://www.blujuicz.com/iphone-adapter-and-cables/rebel-iphone-headphone-adapter-cable.html