Look at this link: http://ubuntuforums.org/archive/index.php/t-4037.html
You have to re-enable gnome-power-manager in System->Preferences->Session
Try it before: run "gnome-power-manager &" in a terminal then click the button: if it is all right it will work.
I'd the same problem in Ubuntu 8.04 with Gnome 2.22.3
Ooopss, just now I noticed that the previous msg was send in 2003
Maybe this answer will help some other people!