This is Definitly a Problem the Lumia 800, 900 and 920 Face if you use a Case and Dust (and other Particles) come in.
My loved Lumia 800 was in the Original Case for Month... After i took it out it looked much bader then the Phones i saw in this Post! :-(
I think cause the Original Covers Fit so good the Problem here is happening faster cause if there is no Space between Phone and Cover...
Cause of my Experience with my Lumia 800 i Remove the Cover of my 920 every 2-5 Days and clean all Dust and dirt out. So far so Good (Using my 920 since December with Multiple Covers. Original and others)