The Tascam iM2 is a stereo condensor microphone that plugs into the 30-pin connector of an iPhone 4/4s, iPod Touch 4G, and iPad 1/2 (possibly the 3, but I can't confirm this). This can turn any of these iOS devices into a portable podcasting recorder.
Since the mics are condensors, I recommend recording in a silent environment, or somewhere with great ambiant noise.
Consider upgrading to the iM2 Kit with Topper for a windscreen/pop filter for $69.95.
[Prices updated 2-12-2013]