I just noticed a neat trick in Entourage 2008: if you drag a folder out of the folder list, and onto your desktop, it creates a .mbox file just for that folder.
This is a good way to move emails from Entourage to many other formats, especially if you're only concerned about a single folder.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008061606245268