The issue that we discuss in this thread has nothing to do with heavy used parts of the screen. As I mentioned in my post I have had my n800 for couple of weeks only, meaning 2-3 weeks, and it's 2-3 weeks of rather light use. The touchscreen sensitivity issue has been there from very beginning and I have strong feeling that it is getting worse very rapidly.
Last week I checked my friend's device which came together with mine in one delivery and its screen has exactly the same problem. I went to Nokia Flagship Store in Helsinki just to ensure that it's not just my device. I checked three devices from store's exposition and every single one of them had exactly the same problem.