I very much suspect that bug #502140 is related to the upgrade of pam. While looking at PAM debconf templates, I see this:
Description: Services to restart for PAM library upgrade: Most services that use PAM need to be restarted to use modules built for this new version of libpam. Please review the following space-separated list of init.d scripts for services to be restarted now, and correct it if needed. . Some other services such as xscreensaver, gnome-screensaver, and xlockmore cannot be restarted for you. You will not be able to authenticate to these services until you restart them manually. So, apparently, that can cause nasty situations like the one Michael experienced during his upgrade tests.... --
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