A backlight lamp is just a small neon bulb. It works or not. At the end of it's life the lamp can display a pink color when the internal neon gas is altered by use . If you look closely at a backlight assy on a iBook or PB you'll notice that it's only a small neon bulb with a wire soldered at each end. These wires connect to the inverter board who gives the power to the lamp. There's absolutely no electronics in the backlight assembly (just a lamp, two wires, one connector that's all). The LCD screen electronics has nothing to do with the screen lightning. The backlight lamp intensity (brightness) is controled by the inverter board who receives his orders from the logic board when you decide to modify it via the keyboard or via the system preferences.
http://www.thebookyard.com/images/pbal12...
http://imgs.inkfrog.com/pix/yallzuan/NCC...
In your case the problem is probably the inverter board or the inverter cable. I guess the logic board could also be the culprit since he manage the whole lightning stuff but the first part I would replace would be the inverter board.