I haven't done much in this area, so I don't know much about what's available for enforcing strong passwords. I did find an article about password security
on Redhat's site which gives some suggestions, though. There are some password-cracking programs you can run to test for weak passwords.
Also, the passwd utility does allow you to enforce password aging, which makes users change their passwords periodically. You can set them to expire with whatever lifetime you like. passwd will warn users if their password is too short or doesn't contain numerals/punctuation, but I don't think it'll actually force them to use a secure password.
Sorry I can't be of more help.