Prevent external monitors from shutting off when laptop lid is closed? [duplicate]
All that needs to be done is to edit the following key and set it to "nothing":
gconftool-2 --type string --set /apps/gnome-power-manager/buttons/lid_ac "nothing"
Then, magically, nothing will happen :)
The top answer to this question does a good job of explaining how to do this for both AC and battery modes, along with instructions on how to change it back to blanking the screen: How can I tell Ubuntu to do nothing when I close my laptop lid?