Screen not locked when coming out of suspend/hibernate

I use Ubuntu 12.04 running in Unity 2D mode. I used to put my PC into standby mode over night but since updating to 12.04 I get no login screen after wake-up. Instead, the session simply continues. Which is quite disturbing since everyone in the company could turn on my PC using my session withoug having to log in.

See also: https://answers.launchpad.net/ubuntu/+question/202397

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gnome-screensaver 3.4.1-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-26.41-generic 3.2.19
Uname: Linux 3.2.0-26-generic x86_64
NonfreeKernelModules: talpa_vfshook talpa_pedconnector talpa_pedevice talpa_vcdevice talpa_core talpa_linux talpa_syscallhook nvidia
ApportVersion: 2.0.1-0ubuntu11
Architecture: amd64
Date: Mon Jul 23 14:00:41 2012
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
GsettingsGnomeSession:
 org.gnome.desktop.session idle-delay uint32 600
 org.gnome.desktop.session session-name 'ubuntu'
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
ProcEnviron:
 LANGUAGE=de_DE:en
 PATH=(custom, user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-screensaver
Symptom: security
Title: Screen not locked when coming out of suspend/hibernate
UpgradeStatus: Upgraded to precise on 2012-05-29 (55 days ago)
WindowManager: No value set for `/desktop/gnome/session/required_components/windowmanager'