Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=662747

Relevant settings:
org.gnome.settings-daemon.plugins.power lid-close-ac-action 'blank'
org.gnome.settings-daemon.plugins.power lid-close-battery-action 'blank'
org.gnome.desktop.screensaver lock-enabled true

With these settings, closing the laptop lid should blank the screen,
which should in turn lock the screen, such that reopening the
lid reveals an unlock dialog rather than the user's unsecured desktop.
However, this does not occur: closing the lid blanks the screen but does
not lock it, and reopening the lid gives access to the user's account.
By contrast, leaving the system idle until the screensaver kicks in
results in a properly locked screen.

I can reproduce this with a freshly created user account, just by
setting the first two keys above (lid-close-{ac,battery}-action);
lock-enabled already defaults to true.

It looks like gnome-settings-daemon needs to emit a lock
signal as part of blanking, as it currently does when suspending.