Its enviornmental. If happened to one phone , its probably going to happen to the next one.
Environmental as in you live in a constant dust storm? Or a lint bubble?
Anyhow I agree with the other chaps, if you pocket your phone - think...pocket lint...goes in from the ear piece.. use a case or sleeve.
I have to disagree with the people telling you that the dust is a result of keeping the phone in your pocket. I've been keeping my phone naked in my pocket since launch, and I've had no dust problems whatsoever.
Another thing to consider - has the dust been there since you got it, or has it been accumulating over time? If it seems like it's accumulating, it will probably keep getting worse. In this case, I'd certainly exchange it.