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 :
There is also a bug report for the radeon driver :
And in launchpad for ubuntu :
You can read this reports and tell me if you have an idea to solve this