I have a Belkin mic that is many years old now that plugs in to my 3G iPod and it records mono sound at 8KHz and 128 Kbps. In my experience that's just not good enough for anything but short, personal notes.
The newer iPods can apparently record in much better quality. There's
this thing, for example, which seems pretty cool.
Looking at
Griffin's iPod products I see the
iKaraoke, but it doesn't seem to be able to record to an iPod, or am I just missing something?