It actually is an input as well for a miced headset but its mono and probably limited range. You'd need some sort of adapter to connect up right as well as attenuate down to mic level. Then any voice recorder app would work. It's not worth it IMO, a laptop would do a much better job and not need anything weird to hook it up.
I used to have a creative 1gb flash mp3 player that did line in recording, if your looking for more portable than a laptop (size of a zippo) I bet those are real cheap on eBay. Think it was called a Zen nano.