A few years back I hacked up a perl chkpassldap.pl script to authenticate against ldap, and optionally authorize against groups and/or ldap attributes. This works with both qmail and Dovecot, and it also supports a somewhat generic method to alter the environment suitable to use for Dovecot as a prefetch userdb.
Anyway, I have been running the script on a qmail-ldap and Dovecot 1.2 proxying cluster using the stock qmail-ldap attributes without any troubles, and figured I would finally release it in the thought someone might find it useful.
http://fritz.potsdam.edu/projects/chkpassldap
I tried to document the ins and outs of userdb configuration. If anyone finds anything that could use correction or clarification, please let me know. It has been awhile since I was in the middle of it all.
Cheers, -Jeff
-- Jeffrey Hardy Network / Systems Administrator hardyjm@potsdam.edu