Well, I don't think it's very bad logic. Form factors are going through an evolutionary process. Mobile phones and PDA type devices and UMPC:s are all evolving and have been around for quite some time. There is vast amounts of consumer research data already available about preferred device sizes. Device size is one part of the puzzle in which devices users prefer. Sizes that people prefer sell more than those that people don't prefer.

In my mind the mobile device sizs actually the most important figure, the screen size is a secondary issue. For screen size people want the display to be "as big as possible" within the given size of the device, unless they want hardware buttons in the same surface or there are some other ergonomic reasons for not doing so. In five years perhaps the screen covers 99% of the surface where it currently still requires some margins.

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I don't think jet packs make any sense.