Greetings. I think this is a bug, but I thought I'd check in here before filing a report. I've been away from Linux for a while so maybe I'm missing something.
I cannot control my LCD backlight. I have a Mac PowerBook G4 (PPC) (PowerBook6,4), which uses the Nouveau driver on a nVidia GeForce FX Go5200 (NV34M). I'm using Fedora 12 (2.6.31-162), with all the yum updates as of 08 Dec 2009. KDE reports 0% when I press either backlight function button; the screen brightness slider in System Settings > Advanced > Power Management > Edit Profiles > Screen will slide, but nothing changes. pbbuttonsd doesn't work, even using pbbcmd (where LCD brightness returns as -1) (I installed the latest RPM I could find, v.0.7.2, but have now removed it.) I tried a nomodeset kernel option in yaboot, and there was no backlight control. I also tried using randr settings, which said something like that no device had that option. The backlight function buttons both register presses, according to lshal -m.
The backlight will shut off when it idles for a period of time. Also, the backlight does not come back on after resuming from sleep/hibernation. (I'm not too worried about that--I'll just shutdown until the backlight control works.) However, the screen will not go to partial backlighting at any time.