Quoth Patrick O'Callaghan on Sweetmorn, the 54th of The Aftermath:
Alternatively, you could jailbreak the phone and copy files using scp. I don't know if the phone will then recognize them as something it can play, but I wouldn't bet on it.
I was able to use gtkpod[1] on a jailbroken iphone to transfer music, manage playlists, photos, etc.
Footnotes: [1] http://www.gtkpod.org/about.html