Comment 6 for bug 317640

If you upgraded, you hould still have some older kernels installed. If you boot and press "esc" you get into the boot menu ("grub") where you can select older kernels. Start with 2.6.27-something, which is the Ubuntu 8.10 Linux kernel, check if it works again, and if so, please work your way upwards to the latest Jaunty kernel and check where it breaks.

If even the Intrepid kernel does not work any more, then it is a regression in a different component. In this case, can you please check https://wiki.ubuntu.com/Hotkeys/Troubleshooting whether the brightness keys produce a proper keyboard event? (you should see it in "xev" already).