My google+ friend Bruce Turner recommended a Comprehensive iPhone Mic Adapter cable for hooking up an XLR mic to the iPhone. IMO this is a great plug-and-play solution for getting great audio into the iPhone. Especially for interviews in the field. Put your iphone on a tri-pod using The Gliff (another must-have iPhone 4 or 5 accessory) and then plug in this cable. Plug in a Shure SM58 and you're good to go. You do need to speak right into the mic but this is a good thing as this will help keep background noise down.
BEST OPTION FOR GREAT AUDIO (lavalier) into your iPhone: Use CablesOnline 6in 3.5mm StereoTRRS 4-Pole Male to TS Microphone iPhone Adapter, Black (IP-MIC2K) to hook up a Giant Squid lavalier mic to an iPhone. Special Note: I've found that it's best to lock the iPhone down on a tripod and then plug in the mic adaptor and then launch the camera app. Buy the Giant Squid at: www.giantsquidaudiolab.com/gs/gs-mono.html
You can also use the Giant Squid mic with a ZOOM H1 recorder.