I doubt that the next iphone's screen would be bigger 'cause that will affect the Retina display

, unless they double the current resolution but the ppi by then would be monstrous (nearly 600ppi)

. But such high resolution on such a tiny screen is not really practical, is it?
Also the iPhone 4's design is nearly perfect, I dont think there will be much change anyway.
Still, if the 6th Iphone have this design. I would buy it without a second thought. Using either ipad 1 or ipad 2 design for the iphone would be awesome
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Judging by the recent release of the new Ipad, iPhone 5 (or 4Gs

) would have the same spec anyway, Apple A5x, Improved Siri, 1gb Ram, 16-32-64 version, 4g LTE, 12MP camera I guess