If your laptop is giving ACPI Brightness error when you press F4 and F5 to higher/lower your brightness and you get no response (Ubuntu 10.10 Maverick) then do the following:

sudo add-apt-repository ppa:kamalmostafa/linux-kamal-mjgbacklight
sudo apt-get update

Then add the updates that show up in Update Manager

Then edit your Grub and add these kernel parameter:

sudo gedit /etc/default/grub

add: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

(that line should already be there..just edit it)

run: sudo update-grub

Reboot > and your brightness levels should work