There is a bug (I think it is) in hardy, where the brightness button is not working on some laptops including Dell and Compaq/HP. The bug report is here and the proposed fix (for the compaq/hp laptops) is to blacklist the video module to let hal do it's work.

I try it yesterday, but it doesn't seem to work. Anyway, the other way to change the brightness in your laptop is write by hand the /proc/acpi/video/VGA/LCD/brightness file.