I'm on a Asus, and I'm having the same problem. I made the grub mod suggested above and rebooted, but the problem reamined.
Here is the laptop's montior information.
Any help would be greatly appreciated. As the OP stated, the display can be dimmed in the System Settings, but the keyboard command does not work (dim or brighten). Interestingly, the keyboard commands for key backlight works as does turning the screen off entirely.