When I ran gnome-screensaver in debug mode it seems that it cannot get the focus, so it bails. Log below:

[main] gnome-screensaver.c:86 (11:19:14):	 initializing gnome-screensaver 3.4.1
[init_session_id] gs-listener-dbus.c:1473 (11:19:14):	 Got session-id: /org/freedesktop/ConsoleKit/Session12
[gs_fade_init] gs-fade.c:920 (11:19:15):	 Fade type: 3
[set_status] gs-watcher-x11.c:346 (11:19:15):	 GSWatcher: not active, ignoring status changes
[gs_watcher_set_active] gs-watcher-x11.c:277 (11:19:15):	 turning watcher: ON
[listener_dbus_handle_system_message] gs-listener-dbus.c:854 (11:19:15):	 obj_path=/org/freedesktop/DBus interface=org.freedesktop.DBus method=NameAcquired destination=:1.1608
[gs_watcher_set_active] gs-watcher-x11.c:277 (11:19:15):	 turning watcher: ON
[gs_watcher_set_active] gs-watcher-x11.c:281 (11:19:15):	 Idle detection is already active
[on_bg_changed] gs-manager.c:596 (11:19:15):	 background changed
[gs_manager_set_lock_active] gs-manager.c:168 (11:19:20):	 Setting lock active: 1
[gs_grab_grab_root] gs-grab-x11.c:536 (11:19:20):	 Grabbing the root window
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:20):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:20):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:21):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:21):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:22):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:22):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:23):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:23):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_nuke_focus] gs-grab-x11.c:385 (11:19:24):	 Nuking focus
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:24):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:24):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:25):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:25):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:26):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:26):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_keyboard] gs-grab-x11.c:153 (11:19:27):	 Grabbing keyboard widget=B8
[gs_grab_get_keyboard] gs-grab-x11.c:168 (11:19:27):	 Couldn't grab keyboard!  (AlreadyGrabbed)
[gs_grab_get_mouse] gs-grab-x11.c:188 (11:19:28):	 Grabbing mouse widget=B8
[gs_grab_get_mouse] gs-grab-x11.c:188 (11:19:29):	 Grabbing mouse widget=B8
[gs_grab_get_mouse] gs-grab-x11.c:188 (11:19:30):	 Grabbing mouse widget=B8
[gs_grab_get_mouse] gs-grab-x11.c:188 (11:19:31):	 Grabbing mouse widget=B8
[gs_grab_grab_window] gs-grab-x11.c:489 (11:19:32):	 Couldn't grab pointer!  (AlreadyGrabbed)
[listener_active_changed_cb] gs-monitor.c:292 (11:19:32):	 Unable to set manager active: 1
[gs_listener_set_active] gs-listener-dbus.c:265 (11:19:32):	 Active-changed signal not handled
[gs_manager_set_lock_active] gs-manager.c:168 (11:19:32):	 Setting lock active: 0
[gs_monitor_lock_screen] gs-monitor.c:232 (11:19:32):	 Unable to lock the screen