I like to rip audiobook CDs to iTunes. In the past, AAC files ripped this way would not display as audiobooks without direct editing of the file suffix. iTunes 8 now offers options to make the process easier.
To convert an AAC file to audiobook format, select the files you want changed, select File » Get Info, and select the Options tab. Click the Media Kind pop-up menu, and then choose Audiobook. Selecting Remember Position is also useful for Audiobook files -- it will restart the audio where you left off if you stop in the middle of playback.
These options only seem to be available when multiple files are selected first.
[robg adds: You can do this for individual files, too -- just look on the Options tab of the Get Info dialog.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081109153600112