The fault lies in between the gap between the glass screen and the body of the phone. You probably can't see it now, but you will, when stuff starts getting in there. Unless you can find a case that guarantees an airtight seal, dust will still find its way in. IMO you got your phone for 30 bucks, just use it nude and enjoy the hardware.
Well that sucks. At least it's still an awesome phone! I might still get a case for it, though, since I'm protective of my electronics (especially when it'll cost $500 to replace it if it breaks).