I have got a new screen too due brightspots.
2 a 3 months later they came back at the same spot.
My local Nokia care point says it is due the design of the phone and the "heat" from the device. They also say the problem will come back, even after a new screen....(They have gotten a lot of return N97's and N900's with this problem) N97 has the same problems.
The newer Nokia phones with amoled displays don't have these problems...

To bad that only after 3 repairs youll get a replacement.....