I'm not sure this is so much a hint as it is a fix. By default, iTunes 7 does not appear to be recognizing all audiobooks as audiobooks so that they are listed in the Audiobooks Library (and the Audiobooks playlist on the iPod). This topic has recently been discussed in multiple threads in the iTunes support discussions. The fix is somewhat buried in this thread, although it is not detailed in a single post but spread over several.
First, the tracks must be AAC. Then you need to make sure they have a file type of 'M4B ' -- of course, Doug has a handy script to do this for you. However, even when an AAC file has the file type 'M4B ' and a file extension of .m4b (which isn't necessarily required), the books still may not show up in the Audiobooks playlist or Library.
All you have to do at this point is delete the tracks from your iTunes library (do not delete the actual song files). When you delete a track from the Library, it will ask if you want to keep the files or move them to the trash. Keep the files. Now import them back into iTunes. Voila, they should show up in the Audiobooks playlist. From what I gather, this is the only fix that has worked for many people (including myself).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060914185019161