(assuming we use a bbs)
PHP BB (http://www.phpbb.com) seems to be the natural BBS system for Wikipedia to use, since it's GPLed and PHP+MySQL. It may be more ornate than we need, but with a little work to coordinate it with the main code, it would be very nice.
The only crucial thing we need to do is to have people who have logged into Wikipedia (any of the language instances) automatically be logged into the BBS.
The simplest way would seem to be to import user info from the current dbs into the BBS user db, and have code to keep the BBS user db in sync with new user creation.
Is there a test server on which this could be installed to work on?
--tc