Here is a mini capsule microphone for iPhone and iPod touch running iPhone OS3.0. The microphone is felxible that it can be bended and turned to the desired direction for better sound reception.
The Flexible microphone works with iPod nano 4G and iPod classic as well. It costs $14.
Another mini iPod microphone that is less flexible.
Tascam iM2 Stereo Microphone for iOS Devices
Thu. Nov 24th, 2011
Blue Microphones Mikey Portable Recorder for iPhone and iPod
Wed. Jan 6th, 2010
SwitchEasy ThumbTacks iPod microphone
Thu. Mar 19th, 2009