If you installed Ubuntu 12.04 on a Macbook Pro 5,5 probably your backlight control stopped working, and you are stuck at the maximum light level.
First of all try the workaround proposed by Ubuntu Community Documentation (https://help.ubuntu.com/community/MacBookPro5-5/Precise), so download the package nvidia-bl-dkms (http://www.ubuntuupdates.org/package/mactel_support/natty/main/base/nvidia-bl-dkms) and install it, then reboot. Is it working?
If not, to fix this, you need to remove the proprietary driver installed via the "proprietary driver" tool in Ubuntu and install them yourself.
Download the driver for nVidia 9400M (www.nvidia.com) and logout your user. Press CTRL+ALT+FN+F2 and log in at the terminal prompt.
Stop the X server with sudo stop lightdm. Install the NVIDIA driver with sh NvidiaDriverYouDownloaded.run. Reboot. If the light control isn't working re-install nvidia-bl-dkms as before.
Now your backlight should work fine.