It looks like the Dovecot module is inserting .99 style directives into the 1.0rc3 version on Ubuntu 6.06. This prevents it from firing up, with a "error in dovecot.conf line 648 unknown userdb" error.
Here's the thread:
http://www.virtualmin.com/forums/message-view?message%5fid=79767
Status:
Closed (fixed)