This problem seems to be related with what I posted in Bug #21478 : according to http://forums.gentoo.org/viewtopic-t-439845.html rivafb must be enabled instead of ofonly (openfirmware) framebuffer driver in order to be able to change display brightness on nvidia-based 12-inch display powerbooks. So rivafb module must be put into the initrd or otherwise compiled into the kernel image. Hope this helps.
-- brightness controls not working for 12" powerbook https://launchpad.net/bugs/35992 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
