
There's also a bug in launchpad that I raised when Gutsy was released. The current work-around is to downgrade the xserver-xorg-ati package to the previous version (if you are running Hardy).
And if you are running Gutsy and not using the laptop LCD and the external monitor at the same time,
then running the following command in a terminal will help:
$ xrandr --output LVDS --off
Hope this helps!