Comment 6 for bug 1027061

If this is any help, here is the installation log (/var/log/apt/history.log) from the package upgrade that changed the config file:

Start-Date: 2012-07-20 06:39:28
Commandline: apt-get -qq -y upgrade
Upgrade: postfix:amd64 (2.9.1-5, 2.9.3-2~12.04.1)
End-Date: 2012-07-20 06:39:31

This is not an issue with puppet, as puppet was not involved in the package upgrade at all. The upgrade was done by a nightly cron job that uses only apt-get.