Comment 3 for bug 773415

A workaround: Pressing Ctrl + Alt + Fn as well as F10 or F9 (in my laptop a HP 6735b, or F1 to F6 in other laptops) to increase or diminish the screen brightness.

I found it at: http://www.tainguyen.com/linux/2011/05/01/it-is-finally-here-ubuntu-11-04-released/