thefunkymunky
macrumors 65816
This may or may not help. Right click on brightness.exe in your System32 folder and select Properties. Tick the box "Run as administrator". For good measure, you may want to select compatibility mode with XP as well. When you double click this now, it should come up with a dialogue box asking whether you want to proceed, which, of course, you do. You should then see the brightness icon appearing in the system tray.
No guarantees, but I think I had to do this to get this and other reluctant programs to run under Vista due to the increased security built in.
Unfortunately. That didn't work for me. No matter what I try "brightness.exe" refuses to run. grrrrr.