OK, I think I've found a relatively easy way. @Ben's suggestion that you can change the type doesn't work for podcasts but set me thinking so thanks for the mental nudge.
The podcast files in question are MP3s. If you convert them to AAC (right click the podcast track in iTunes and select "Create AAC Version") then the new version appears in the regular library as a music track.
At that point you can do "Get Info" > "Options" and change the MediaKind to AudioBook and you're good. Note that you can only change MediaKind when editing tracks one at a time, you can't bulk change it.
Not sure what happens if they're already AACs. It's possible that you can change them anyway (though I don't think so) or you could convert them to MP3s (same method, then convert them back if you want AACs) and if could have the same effect. Might lose a bit of quality in the process but they're probably spoken word so it's probably not too much of an issue.
Depends whether this is intended behaviour or not I guess. If it's not then please Apple don't fix it!
(works on iTunes 8.2)