This used to work. You can even see that the iPod still has an “Audiobooks” section in the source list. But iTunes 9 removed the options for syncing audiobooks, and won't let me add any.

The audiobook I'm dragging is an MP3 file. I also tried (not in the video) dragging audiobooks purchased on iTunes, and those didn't work, either.

UPDATE: Never mind. Turns out I had the checkbox for manually managing music (and audiobooks) turned off. I used to have it on, which is why it used to work. No bug.