Eric Poelzl found the solution for those of us that can't mount CDs or DMG files. He has posted it in Apple Discussions. Open NetInfo Manager (in Applications/Utiltiies) and see if there is a Unknown User (click on 'Users' in the Directory Browser area). If not, you have to add it back in. Create a new entry with the following settings:
realname: Unknown User
expire: 0
name: unknown
uid: 99
passwd: *
home: /dev/null
change: 0
shell: /dev/null
gid: 99
Save your changes and you should be able to mount your CD's and DMG files again.
[Editor's note: I have not tried this myself, and be careful working in NetInfo; if you mess something up, you could make your system unusable.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010727045852728