My 700 also is getting more and more dust particles, most of them are at the top of the screen. I did try compressed air, I was not able to move particles. I use the reader at least 2 hours a day in pubic transportation, it's not a dust-free environment. I notice these particles changing their locations almost daily. One day it was so bad I couldn't concentrate on reading. Next day, there were only few left.
My first unit had a hardware problem (
https://www.mobileread.com/forums/showthread.php?t=34851), I replaced it in SonyStyle, new unit got exact same problem after 3 weeks. There is no single scratch on the reader, it's never been dropped. I cannot take to the store again, too many days past. Now with this open space on the device, it will collect even more dust.
While I like the 700, it's been disappointment to deal with this problem plus dust, in such a short period of time of ownership. My original 500 had no problem during a year, current 505 has been very solid for many months. Of course, layers of screens already mean a potential dust problem. But my two units haven't seemed to be well-built devices.
I'd like to hear from SonyListents too.