iphone3ger is completely right. Apple had an issue were 10% of their iphone power adapters would over heat, and they actually recalled quite a few. The green dot indicates the adapters were fixed.
The dot on the adaptor is seperate from the one on the box. The green dot on charger started last year with the 3g. He means the dot on the box. All chargers have a dot. Not all boxes do.
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Green dot in back of iPhone 3gs box?
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