The audiobooks are all on my device, but the playlist is not. I don't have iTunes U or the Apple podcast app on my iPhone.
I find that I like to use playlists for audiobooks. With my car iTouch, it will automatically bring up the next book and start it after I finish one without me having to take my eyes off the road to pick a new book. With my iPhone and iPad, I simply like being about to group books together so I can easily decide what to listen to based on my mood. I've got play lists for fiction, non fiction, and several of my favorite authors.
It appears to me that Apple doesn't recognize playlists that have non iCloud entires in them. I was able to mess around a bit with the iTouch and create a new playlist, then copy the audiobooks into it. I haven't been able to get the iPhone to do the same. What's funny is that I've been in the ios 6 beta and didn't have any trouble until they rolled out the gold master, so this is something that got introduced at the last minute.
I've looked at the audible app, but that app doesn't support playlists. It's a bit like Amazon's kindle app in that it only supports sorting by author, most recent and title. I have no idea why this is such a blind spot for Amazon.