I don't know if this is relevant to the original problem asked.
I got the winning bid on a 20" imac 2.0 and went to pick it up, When it was brought out and turned on I noticed dark patches at the corners of the screen and mentioned it to the seller, who promtly walked into the back of the store and took out another imac, turned it on and no dark edges to the corners.
I think this points to variations in the screen manafacutre or rather bad quality control at the least.
So I was wondering if the dark half is really a back light issue and not just a LCD issue.