The other day I was trying to help a relative make the switch from OS 9 to OS X (he hadn't been using OS X too much), and he wanted his e-mail set up in Mail.app and his addresses in Address Book. I got the e-mails over fine, but the addresses were giving me trouble.
Outlook Express for OS 9 seems to export contacts in a non-standard format (go figure). It looked like it was LDIF format, but I don't think it was because Address Book was refusing to import it. I was trying to find programs that would do the import, but then I found an easier way:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030109063216844