I recently installed Arch Linux on my Macbook Pro. In order to control the screen brightness I installed nvidia-bl from the AUR (I use yaourt so: yaourt -S nvidia-bl). This sets up a folder in /sys/class/backlight/nvidia_backlight and in this folder you can get and set the screen brightness via the file brightness:

> cat brightness
1000
> sudo su
# echo 500 > brightness

But after a system update (pacman -Syu) the /sys/class/backlight/nvidia_backlight folder was gone and I could no longer change the brightness.

By running dmesg I could see that the nvidia_bl kernel module was not being loaded during boot:

[    7.183486] nvidia_bl: disagrees about version of symbol module_layout

Trying to load it manually also failed:

> sudo modprobe -v nvidia-bl
insmod /lib/modules/3.12.7-2-ARCH/extramodules/nvidia_bl.ko
modprobe: ERROR: could not insert 'nvidia_bl': Exec format error

These errors mean that the kernel module is not compatible with the kernel. This can be solved by recompiling the kernel module. Since I use yaourt I can simply reinstall nvidia-bl: yaourt -S nvidia-bl.

After that you can manually load the kernel module again:

> sudo modprobe -v nvidia-bl
insmod /lib/modules/3.12.7-2-ARCH/extramodules/nvidia_bl.ko

And you can now control the brightness again :)

Note: I use a Aluminium Unibody Macbook Pro 5,5 from 2008