A quick workaround that I use in Lucid when I want to leave my laptop running overnight but want the screen off "immediately":
1. Click power management applet and select Preferences
2. Select Put display to sleep when inactive for: 1 minute
3. Wait 1 minute.
4. Activate the screen again as normal, the power management applet is still up - reset the "Put display to sleep when inactive" value to whatever it is normally and click Close.
A quick workaround that I use in Lucid when I want to leave my laptop running overnight but want the screen off "immediately":
1. Click power management applet and select Preferences
2. Select Put display to sleep when inactive for: 1 minute
3. Wait 1 minute.
4. Activate the screen again as normal, the power management applet is still up - reset the "Put display to sleep when inactive" value to whatever it is normally and click Close.