The screen issue appears to be recent, certainly more people are commenting on it. It might be a bad batch of screens.
Here is a link to a thread on the Bebook forum:
http://mybebook.com/forum/viewtopic.php?f=4&t=339
That picture shows exactly what I have although mine seems to change the placing off the dots. You'll notice white dots on black and on white back ground some black dots.
The PRS-505 didn't have this problem, whites and blacks were completely uniform.
I am going back on what I said about Mobi on the Bebook. It just looks wrong, the formatting of headings don't work well and letting is double spaced and large even on the smallest font setting.
For me RTF seems to be the best format so far. It has accurate page counts and if done well the headings can look good.