Whenever possible i'm still a big fan of capturing audio from a subject using just a Zoom H1 portable audio recorder and a simple Lav mic. In fact I own (5) Zoom H1 recorders. No worries about interference or audio drop outs, and it's a very inexpensive way to capture high quality audio.
But just in case you don't happen to have a Zoom H1, here's a video posted by AspenMics that shows you three different FREE Apps from top audio companies Rode, Tascam, and Zoom that you can download to turn your iPhone (or ipod touch) into a Portable Pocket Audio Recorder.
Keep in mind that not every Lav microphone will work when plugged directly into an iPhone. Some microphones will need a special TRRS adapter (or other). But if you happen to have a compatible lav mic for your iPhone such as the Rode SmartLav+ or AspenMics HQ-SPK, then these apps should work with either one.