All distributions I tested it on my notebook, it's the same problem.

the only distribution that this problem does not occur, is Gentoo.

I have no problem adjusting the brightness on KDM.

see the problem in the video below.

http://youtu.be/LGs_LUqTBgY

It appears that two processes may be fighting over the brightness controls. In System Settings > Shortcuts & Gestures > Global Keyboard Shortcuts, select "KDE Daemon" as the component, and then set both screen brightness actions to "Custom: None".

After completing that, are you able to change the screen brightness normally?