Comment 28 for bug 535097

Workarounds:

Desktop: just hover your mouse over the icon and use the middle scroll button to raise/lower brightness (don't click because the drop down slider doesn't work)

Terminal: cat /proc/acpi/video/UVGA/LCD/brightness
(you may have to look around in /proc/acpi/video)

It will look something like this (probably with different numbers):
levels: 20 24 28 32 37 43 50 59 69 81 95
current: 95

Pick a number and then:
sudo su
echo THE_NUMBER_YOU_PICKED > /proc/acpi/video/UVGA/LCD/brightness
exit

for me brightness changes immediately