I’m posting this to help anyone that has the same problem I did. The problem was that my Lenovo G580 screen stayed at max brightness after installing Ubuntu 12.04 LTS (32bit). The brightness up / down buttons did nothing. During the day it wasn’t much of a problem but in the evening the screen was too bright. The fix was pretty simple:
Edit the grub file:
sudo vi /etc/default/grub
Change this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
So that it says:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Save and exit the file.
Now update grub:
sudo update-grub
Monty Kamath 
thnkx man! even I just loaded 12.04 LTS on my g580, and was wondering hw to solve this problm 🙂
Thank you so much. This worked really well 🙂
no its not working on my linovo G50