I’ve been recording audio for fun using a 2nd generation ipod nano & a couple of accessories, the Belkin TuneTalk Stereo & the xtrememac MicroMemo. (I switched from using the MicroMemo to the TuneTalk because the latter works much better with my microphones – the autogain balances out for phantom power or bias power, I forget.) Out of curiosity, I checked to see how well they work with various ipods:

                MicroMemo       TuneTalk

ipod - 4th gen  no              no

ipod - 5th gen  untested        works

nano - 2nd gen  works           works

nano - 3rd gen  works           works

nano - 4th gen                  works

ipod classic    works           works

iphone          no              no

iphone 3G                       no

ipod touch      no              no

touch - 2nd gen                 no response

There are other accessories that allow you to record – the ivoice (for touch & iphone, apparently) & the Griffin italk pro, at least – but I own neither, so haven’t tested those broadly. The italk pro worked well in a brief test with the nano 2nd gen & my microphones, at least.

Update (12/10): Tried the TuneTalk on a newer generation of ipods.