From Ubuntu 10.10 i noticed that in the power management window the “do nothing” opition when the lid is closed is not available anymore. Although it’s really useful when you connect your laptop to a external tv or screen, to close the lid and watch a movie in the darkness. Likely the shell comes in our help and there is a one line command to add the “do nothing” option and set it.
gconftool-2 --type string --set /apps/gnome-power-manager/buttons/lid_ac "nothing"
This will set a “do nothing” when the lid is closed AND you are connected to the power line.