Interestingly, editing the keys using gconf-editor (e.g. setting '/apps/gnome-power-manager/buttons/lid_ac' to 'nothing') adds the 'Do nothing' option to the drop-down list in Power Management Preferences.
Interestingly, editing the keys using gconf-editor (e.g. setting '/apps/ gnome-power- manager/ buttons/ lid_ac' to 'nothing') adds the 'Do nothing' option to the drop-down list in Power Management Preferences.