Comment 4 for bug 324918

In the old times, the acpid should react on the event defined in /etc/acpi/events/lidbtn and run /etc/acpi/lid.sh. You can verify this by restarting acpid in debug mode:
 sudo pkill ^acpid
 sudo acpid -d
... now close the lid and open it again
Afterwards stop the debug one and restart it normally:
 (ctrl-C)
 sudo acpid

Nowadays, gnome-power-manager is responsible for these things. So /etc/acpi/lid.sh exits because it sees gnome-power-manager running. You can see g-p-m in action like this:
 sudo pkill gnome-power-manager
 sudo gnome-power-manager --no-daemon --verbose
... and close the lid
Restart with
 (ctrl-C)
 sudo gnome-power-manager

However I am not sure how laptop mode is hooked into g-p-m (or should have been) for lid actions.