David'sBlog

Spent a couple of hours trying to fix the screen saver on my laptop. The unlock dialog box refused to appear, so you had to press ctrl-alt-F1, login to the console and killall gnome-screensaver to get back to the desktop.

This FAQ gave me a few hints. I found I was getting errors like:
Unable to establish service gnome-screensaver: Critical error - immediate abort

When I looked in /var/log/authlog I found the line:
Nov 24 19:39:24 localhost gnome-screensaver-dialog: PAM _pam_load_conf_file: unable to open /etc/pam.d/common-password

chmod 644 /etc/pam.d/common-password fixed the problem.  I have no idea how this file ended up with the wrong permissions.