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Go to the genius bar and they should swap it out for you, I've had this problem and they swapped it over.

My replacement has just started developing a lot of dust under the screen so I should probably go back to them.
Just so you know you probably could have found the answer for this question throughout this forum, just search "iPhone Dust" or something :).
 
Ya, I have dust under my iPhone. It's not very visible unless under sunlight or bright light, except for a few particles that are pretty unsightly. Not a huge issue ATM but next time I'm in a town with a genius bar I'm going to see if they can do anything about it. I have the applecare protection so hopefully it will be pretty painless.
 
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