Bug#489495:

[Thomas Champagne]

Hi 
I have the same problem on my Ibook. 
I create different upstream bug reports in many project because I don't
where is exactly the bug. 
I can try to explain you why I don't know where is it. 

The different power-manager like gnome-power-manager for Gnome or
Powerdevil for Kde call the same method that this command : 
xset dpms force off
So, they use DPMS to switch off the screen.
But, when you check the xorg.0.log, you can see this line : 
"No DPMS capabilities specified; RGB/Color Display". 

So, on Ibook, it isn't possible to switch off the screen with DPMS. 
The power screen is managed by the Power Management Unit (PMU). 

I have created bug reports in Gnome power management : 
https://bugzilla.gnome.org/show_bug.cgi?id=550524
There is also a bug report for the radeon driver : 
http://bugs.freedesktop.org/show_bug.cgi?id=19168
And in launchpad for ubuntu : 
https://bugs.launchpad.net/debian/+source/xserver-xorg-video-ati/+bug/249568
https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/117836

You can read this reports and tell me if you have an idea to solve this
bug.