Version:           4.6 (using KDE 4.6.3) 
OS:                Linux

Since KDE 4.6.2 on my laptop, when the AC power is unplugged the screen brightness bar appears with a low value, but physically it doesn't change the monitor light. I can change the screen brightness using the Fn keys.

Also when we try to resume from suspend the monitor stills black without any light.

Reproducible: Always

Steps to Reproduce:
1) Unplug AC power on a laptop with ATI Radeon 5850
2) Resume from suspend to RAM

Actual Results:  
1) The screen brightness doesn't change
2) The screen stills black without light

Expected Results:  
1) The brightness should be reduced
2) The computer should resume from suspend

With KDE 4.4.5 on Gentoo, everything worked well.