I recent upgraded from ubuntu 6.06 to 8.04 - the first issue you fixed was the dovcot problem with permissions - which by teh way is still occurring with afresh install..
now I have that fixed - but neither of the webmail programs work - i cant log in with either roundcube or squirrel
roundcube error
DATABASE ERROR: CONNECTION FAILED!
squirrel error
Warning: include_once(DB.php) [function.include-once]: failed to open stream: No such file or directory in /home/editnet/public_html/webmail/functions/db_prefs.php on line 40
Warning: include_once() [function.include]: Failed opening 'DB.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in /home/editnet/public_html/webmail/functions/db_prefs.php on line 40
it seems that this may be another issue arising from ubuntu 8.. never had this, problem with 6.06..