Hi,
I use dovecot-shared to add read permissions to the group in my mail server, which uses virtual users. This is only to ease backups.
However, a few files are not created with the permissions in dovecot-shared. I've seen this happen to 'subscriptions', 'maildirfolder' (when dovecot-shared exists; the change to correctly create maildirfolder in this case is very recent), and possibly 'maildirsize' (I'm not sure about this one, it might be exim that is creating it with wrong permissions.)
This has been tested in recent snapshot package, from the Debian repository listed at http://wiki.dovecot.org/PrebuiltBinaries. Here's dovecot -n for reference: # 1.1.1: /etc/dovecot/dovecot.conf log_path: /var/log/dovecot/main.log info_log_path: /var/log/dovecot/info.log log_timestamp: %Y-%m-%d %H:%M:%S login_dir: /var/run/dovecot/login login_executable: /usr/lib/dovecot/imap-login login_processes_count: 1 first_valid_uid: 100 mail_privileged_group: mail mail_location: maildir:/home/vmail/%d/%n/INBOX:INDEX=/home/vmail/%d/%n/dovecotidx mail_plugins: quota imap_quota auth default: mechanisms: plain login cram-md5 passdb: driver: passwd-file args: username_format=%n /home/vmail/%d/db/passwd passdb: driver: pam userdb: driver: static args: uid=104 gid=8 home=/home/vmail/%d/%n userdb: driver: passwd socket: type: listen client: path: /var/run/dovecot/auth-client mode: 438 plugin: quota: maildir