my iPhone 3G had a bad seal between the top glass and LCD and was collecting more and more dust under the top glass as time went on. I brought it in to the Apple Store just before the 1year warranty was up, and the Genius replaced the whole display module (glass, touchscreen, LCD display) while I waited, no charge. I know other people who did the same thing.
in your case, if it's just one large piece of dust and you're still in warranty, you might want to wait to see if it gets worse, then bring it to the Apple Store before warranty expires.
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