I have a custom LXDE on a Lubuntu installation. After the most recent upgrade to 13.04 (Raring Ringtail), I realized all my custom Compiz shortcuts were all overwritten. I used to use Ctrl+Alt+L as a shortcut to launch xscreensaver to lock my screen, which worked coherently with Synergy that allows me to use my laptop's keyboard and touch-pad to control other machines in the same network. Not surprisingly after the upgrade Ctrl+Alt+L stopped launching xscreensaver and the same key-combination would fire-up LightDM lock-screen. LightDM's lock-screen is actually quite nice and I would love to keep it if it would work with Synergy, but it doesn't. So I had to disable it. Here's how you do it:
gsettings set org.gnome.desktop.screensaver lock-enabled falseAfter disabling LightDM, I can launch xscreensaver as shown below to lock my screen:
xscreensaver-command -lockHope it helps someone in my situation.