There appears to be a problem with some computers and Ubuntu-based distributions (verified on Linux Mint 13 and Lubuntu 12.10) where you can't adjust the screen brightness. This only applies if the brightness controls pop up and appear to work, but the brightness does not actually change - it's stuck at full-bright. If you aren't seeing the brightness popup, that's a different problem.
Here is how I re-enabled brightness controls on my Acer Aspire One.
gksudo gedit /etc/default/grub
Find this line:
Change it to this:
Save changes and update Grub:
After doing this, I can fade all the way to black. Great for battery life! Hard to read though.