There has been some angst over the iPhone's recessed headphone jack because, it is said, it prevents you from using regular headphones with the device. Some people have even gone so far as to surgically alter their headphones, but the more sane approach is to buy a butt-ugly, but functional, Belkin adapter or wait for the aesthetically-pleasing, but not-yet-available, Griffin version.
However, I say to you, stop! With the cacophony over this issue, I felt certain that I would have to buy one of the aforementioned adapters. I even had the Belkin model in-hand at an Apple Store, ready to buy, until my inner design snob talked me out of bringing such an offensive hunk of plastic into my home.
The inspiration struck. I dug out my Griffin SmartShare dual-jack splitter; I purchased this a while ago for use with an iPod. It works well. Unfortunately, it doesn't work with the iPhone. If you have one, don't bother trying it.
But there is something you should try instead. Try the headphones you already own. As it turns out, to my surprise, my Etymotic Research 6i earphones work with the iPhone. It's not a super tight fit, but its perfectly functional, and they sound great. Hurrah! No adapters or cutting required. It seems that the jack isn't out of reach of all headphones, just some headphones.
Personally, the only time I ever plan to use these earphones with the iPhone anyway is on board an airplane. Otherwise, they don't make sense. The iPhone is, after all, a phone! If a call comes in and you're using regular earphones, how will you answer it? It will be very awkward to do so--that's why the earphone with microphone--which come free with the iPhone is really the more usable solution. But, see me in the air, and I'll be in audio-bliss with my 6i's.