An article popped up on debian-administration.org. I'm going to have to take a closer look at one of the comments:
If this can work well with courier (which I guess it should) then this will solve an issue I've had a while (and I don't want to go the route of a database for this).
Edit: Various things that have come up under discussion
courier assumes $HOME - to change this you need to change the authenticator so that it changes the value of $HOME (at least according to various google searches)
dovecot can look at several password databases (from 1.x up) and they can specify different locations for maildir http://wiki.dovecot.org/Authentication
exim smtp auth via ldap - your starter for 10 here: http://www.alios.org/exim4ldapauth.html
all virtual users need also to be in the virtual file if you use catchall addresses