Comment 8 for bug 646858

I did a fresh install of dovecot-postfix on Maverick, including:
dovecot-common 1:1.2.12-1ubuntu5
postfix 2.7.1-1

I had no problem restarting dovecot:
$ sudo service dovecot restart
dovecot start/running, process 2490

Or upgrading dovecot-postfix:
Unpacking replacement dovecot-postfix ...
Setting up dovecot-postfix (1:1.2.11-1ubuntu1) ...
Configuring postfix for dovecot-postfix integration: .................... done.
dovecot start/running, process 3015
 * Stopping Postfix Mail Transport Agent postfix [ OK ]
 * Starting Postfix Mail Transport Agent postfix [ OK ]

So it looks like it's not 100% reproducible, or only affects busy dovecot instances ?

The classic init script had a do_stop that would run twice in a row "start-stop-daemon --stop --quiet --oknodo --retry=0/30/KILL/5 --pidfile $PIDFILE --name ${DAEMON##*/}"... maybe that's why it succeeds in your case.