Mine has the same problem and it is extremely anoying. I still have the 770 and the screen is much better that the N800s. It does have a pattern (touch layer) that the N800 doesn't have, but the backligh is very regular.
On my N800 I have about 2cm that are much darker than the rest.
I am probably going to try to tak it back. I am usualy not very picky but it is very noticeable in mine.