Comment 15 for bug 354792

The issue is caused by a recent change in gnome-settings-daemon that adds a 30 sec delay for starting gnome-screensaver. This is the reason for it (directly from source code):
         /*
          * We'll start the screensaver after a delay so it doesn't
          * compete with other processes while the session is starting
          */

I've attached a patch that adds a new option to gconf for setting a custom delay timeout value. It defaults to 30 seconds. Setting it to 0 will results in gnome-screensaver immediately started, and screen lockable.