I believe that the Right Solution would be to design an API for the underlying user database. I want to support people who, e.g. already have user accounts through Zope or some other Web-app/portal software, so that they don't have to force users to log in twice. This would let us distribute a GPL compatible database back-end for stand-alone operation, but should also let us integrate with existing user databases.

That's about as far as my own thinking has gone.

If it could be set up so that you don't *have* to go acquire another database to use Mailman, that would be best. MySQL is huge, hard to build, and has stopped me from using four or five Gnu/Linux tools now, and it just doesn't have to be that way.