iPhone 4S Otterbox Defender: Exposed speaker? Advice?
I own the Otterbox Defender Case for the iPhone 4S. The speaker for the earpiece, the speakers on the bottom and the camera lens (but not flash) are exposed to air. The Defender offers some protection because the thickness of the case creates a cavity roughly 3mm deep around the exposed speakers. These cavities DO collect dust. 

When I opened my case to answer this question I found only a very slight amount of dust infiltration within a very close vicinity of the openings "exposed to air". I do not believe this dust will in any way damage the phone or mar the finnish (because the slight dust inside was more like "fuzz" than "grit") but I have only had my case for a few weeks. I have a screen protector on the front of my phone and on the back too, with the Defender case on top of that. 

The biggest annoyance for me with this case is that dust collects in the "protective" cavity for the camera lens and is not easy to clean on-the-go (you need something like a q-tip or rolled piece of tissue) which makes my HD camera take blurry pictures. 

I would recomend Otterbox include a silicone cover for the camera lens like they have included for the charge port, headphone port and silent button -OR- re-design the shape of the cavity to collect less dust or allow the user to wipe the lens with a finger or cloth to clear dust.