Gnome: Leave Display Active When Laptop Closed
Bu default the gnome power management preferences only allow you to select "Blank Screen", "Suspend", "Hibernate", "Shutdown" for the "When laptop lid is closed:" option.
To keep the display showing (for instance if an external monitor is plugged in) this can be overridden on the command line:
gconftool-2 --type string --set ~/.gconf/apps/gnome-power-manager/buttons/lid_battery "nothing"
Or by using gconf-editor (navigate to the same section and set the lid_battery and/or lid_ac value to "nothing"
I found this method here: http://ohioloco.ubuntuforums.org/showpost.php?p=9057882&postcount=8