I'm a two-credits-a-month Audible user since 2006 or so, first with my iPod, now with my iPhone. Until a couple of months ago I used iTunes playlists to manage my audiobooks, but I've switched to the audible app because I sometimes use whispersync for voice(tm). And I don't miss setting up playlists.
As a side note, I would recommend that anyone who listens to audiobooks a lot on a phone (or other device with bluetooth) look into getting some bluetooth headphones - I have a decent pair from Motorola that cost $35 at Amazon. Not having the headphone wires is very nice.