I have a 20" iMac EMC Number 2133 that has the same problem. It is about 3 years old and out of warranty. The top half of the screen is considerably dimmer than the bottom. It just started about 3 weeks ago. I started by ordering a brand new inverter and replaced it in my machine, same problem. I even flipped the inverter around and tested with different plugs into the top backlight and it was still dimmer than the bottom. The inverter I bought was from dvwarehouse.com and was claimed to be brand new, so it seems that the problem is the screen. I dont want to pay $500+ for a new LCD, I'd rather find a backlight and replace it on the screen, or if possible, buy a cheaper 20.1 inch monitor from a big box store and swap the screen out if it is compatible. If anybody knows how complicated/dangerous this is, let me know. Also, it would be great if anyone could point people in the right direction to find parts needed for this repair. (LCDs, Backlights, etc.)
I hope I'm not breaking any forum rules posting this on an existing question, I just wanted to contribute more information to the original posters question.