Sounds like some of the mounts behind the LCD panel are pressing in, causing the lighter/brighter spot. Even if the iBook is out of warranty, try calling Apple and ask for "customer relations" to discuss the product. As I understand it, this has been a problem with many notebooks, and most of the companies have an unoffical recall/replacement.
Years ago, Apple replaced the pastics on my old PowerBook after the mouse button broke. It was almost a year past the end of the warranty, but I argued to customer relations that the problem was an obvious design flaw (it was, very flimsy connection on that one model) and the did fix it for me for free... but I had to talk to about 4 people before they gave in since I was out of warranty.