I highly doubt they'll exchange your phone for something so simple as lint or dust.
Me personally, I Manage in the Fast Food industry (SONIC) and I am around Grease, Dust, People running around & running inside and outside constantly. I worry mostly about getting Grease between my hard keys more than anything. But I keep my phone in a leather hip pouch. It used to wipe the screen clean of dust; before I put a leather case on it (Now it has a space between the case and the screen so now it doesn’t touch the pouch at all)
But with all the chaos at work, no problem keeping the Eris clean of minimal grease and dust. The screen protector is amazing, no scratches at all unlike the protector for my LG Dare I had prior.
So I am not sure what to tell ya on protecting it from dust, all you can do is take care of it. It's an electronic piece of equipment and cost a hundred bucks for most. Not cheap in any sense. Get a sleeve, a pouch, a case or whatever you feel will help protect your phone the best and fit your lifestyle. ^__^