Since a view month the upper part of my iMac-Display (mid 2007) was darker than the lower part. Also the the contrast on the complete display was bad. (see first picture)
After the first try, I didn't have the problem on the upper half but on the lower part (changed both, upper and lower CCFLs, second picture). So the next day I opened up everything again and found that I had torn off one of those thin cables soldered to the CCFL during reassembling.
Now the display works as on the first day again. (third picture)
After removing the black surround (step 21) DO NOT remove those transparent sheets from the backlight-reflector. So you won't need to clean them before reassemble. Just pull out the CCFL tubes.
During reassembling after putting the LCD back onto the backlight-reflector, be sure to clean off the dust from the top sheet with a soft tissue.