(From www.thisismynext.com)
http://thisismynext.com/2011/04/22/iphone-5-design/
In the article, he explains why they mocked it up as so and even went into why a 4" screen would not be possible with the current screen resolution because that would take it out Retina Display territory which is a big part of Apple's branding (and still a resolution that has not been matched by the competition).
I think it makes sense, it looks like the iPad 2 and the iPhone 4 looked like the original iPad. Plus... thin is in!