This happens over and over with nokia devices (770, N800, N810, N900). I had a problem with 770 and N810.
I think the main issue is the resistive technology. It was simply not meant for so much swyping and other finger dragging gestures.

Initially they were intended for point interaction, but someday some clever software developer invented thumb-friendly operation. Actually the wanted to emulate a iFruity which did have proper technology behind it.

It basically means that areas of the screen are systematically pressured, and the thin film becomes less uniform sooner or later. At the same time, covering plastics is deformed as well over time.

Can you specify what is your general use pattern?
Do you use stylus at all?