SuperPhone.cz (via Engadget). This site claims that they have gotten a new iPhone, and have tested it under a microscope.
By doing this, they were able to count the RGB elements of the screen, they discovered it to be exactly twice as dense in the horizontal and vertical directions compared to the current iPhone. If this was true, it would quadruple the total number of pixels, and change the iPhone's resolution from the current 480x320 to 960x640. This would, in turn, make the density 320dpi -- higher than any other device on the market.
Rumors of this new phone resolution first came out in March. The good thing about doubling the resolution is that older apps with smaller resolution would easily be able to run on it as well. This resolution doubling is already present on the iPad.