Cellphones, while great for calling others, aren't quite known for having best of mics. While things may sound reasonable during your average phone call, serious audio recording is another story all together — and the discrepancy between the two can be frustrating. While budding bootleggers might be less than impressed, it's easy to ditch your internal mic for something better.
A few things to keep in mind, however. Mic input via this method is mono only, so those hoping for stereo will have to keep reading. Also, because the input is built for mic-based sources, line level audio will be *much* too loud for the iPhone to handle. That being said, if you're looking to pipe in output from a mixer or other non-mic device with this method, you'll need a way to limit the audio accordingly.
The Alesis ProTrack is probably overkill for most. But still totally awesome. And yes, that's an iPod Touch trapped inside.
Belkin's TuneTalk, Griffin's iTalk and the heavy-duty Alesis ProTrack all support stereo recording via an external microphone, making it easy to record great sounding audio with apps that support it. While its unclear whether the iPhone 4 accepts external inputs with its video capture app, there's potential for some impressive sounding videos when combined with 720p shots.
Combined with the plethora of decent audio-recording apps on Apple's marketplace, you can easily turn your iPhone into a serious, low-cost recording machine — with both mono and stereo inputs too. The only thing we don't know is whether any of these inputs can, in fact, be turned up to 11.