Scratch that last comment. got it working in KDE 4.2 on MacBookPro3.1
I already have Gnome installed, and already had that _bl line. the problem is that KDE shortcutted the brightness commands already to something else. So, just go into preferences > keyboard & mouse > Global Keyboard Shortcuts > Guidance Power Manager (in the dropdown for KDE component) and then change the shortcuts to something you are happy with.
I switched mine to F1 and F2 because I havent used those keys since the WinX days.
Viva KDE4.2 !! it looks really pretty so far. Hope it is stable.