Please read this article for background/more info: iPhone/iPad external microphones
Using this setup, I am able to record interviews on the go on my iPhone, without carrying around a mess of equipment. This rocks. Here's a picture of my entire on-the-go interview kit:
Read more about setting up an iPhone 4 with a Lav mic, or other line-level inputs and microphones. Also, find how to create your own iPhone 4 tripod adapter/case.