Yes, I have noticed them as well. My best guess is that they have something to do with the way the digitizer is laid out on the screen. The screen is divided into areas by the digitizer which lays on top of the LCD. The circuity that makes the digitzer work is usually invisible but for this phone it seems otherwise.
Here is a good explaination as to how it works, this is standard for most if not all touch screen phones: http://www.ifans.com/forums/threads/how-the-touch-screen-works-for-all-whos-ever-wanted-to-know.83706/
I wouldn't worry about it. Just one of those things that most of us happen to notice.