I wanted to use an external microphone with my iPhone, so I could record audio from an event. Unfortunately, there aren't any cables that give you a real microphone input jack. Fortunately, I was able to find an unexpected solution: the Mini A/V cable that came with my digital camera!
On one end, it has the three standard RCA plugs found on the front of every TV. On the other end, it's got a four-conductor mini plug that's the same as the iPhone's headphone jack. Just plug the cable into the iPhone's headphone jack, and use the RCA jacks like this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090501144335832