Install "Brightness" by bergdesign.
http://bergdesign.com/brightness/
Brightness adjustment can then be made via the menubar.
It shows that IntelBacklight.kext is not completely broken... (we have plenty of time to investigate/fix, if necessary)
-OR-
Remove IntelBacklight.kext and use AppleBacklight.kext.
Keep in mind to make AppleBacklight.kext work 100% correctly requires specific ACPI patching and patches to AppleBacklight.kext usually via custom AppleBacklightInjector.kext.