Comment 2 for bug 446191

*Sigh*, gnome-session is using "/apps/gnome-screensaver/lock_enabled" to determine whether to lock the screen or not, and this is set to false on Ubuntu. I see from https://bugs.edge.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/428115/comments/44 that you haven't changed the preference from the default, so this won't work for you