The initial problem: I can't change the brightness of my laptop screen with Ubuntu 13.04.
xbacklight -set 41(for a moderate brightness)
Problem: Didn't work. Xbacklight did not have any function, no matter what argument was given.
Solution: Search the line within /etc/default/grub and change it to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor"
sudo update-grub, xbacklight works as intended, and the screen brightness keys on my Laptop also work normal. Very nice.
If your screen brightness changes again after reboot, you can enter the xbacklight line to the startup applications. After login, your eyes will thank you.
Update: Better late than never... added file name to edit and minor text changes to clear things up :)