Apple Mail v2 does not use the .mbox format as the Panther version did. At least, that's what it looks like.
While trying to move my mail to Mozilla Thunderbird, I could not find a way to export my mailboxes. However, if you open Apple Mail and drag and drop a mail folder onto the desktop, the folder is called Foldername.mbox, with Foldername being the name of the Apple Mail folder. I had no trouble importing this folder into Thunderbird.
[robg adds: This hint covered another way to do this, though it relied on a third party program, and included some fixes for potentially troublesome emails.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060326110248854