THis of course would be a textbook implementation of Marketing theory. First you get your product noticed and talked about (iPhone 1). Then you launch a new, improved and different looking product (iPhone 3G). Then, just as car makers put a new engine into a tired looking design, launch a new version with better internals (iPhone 3GS). Then when the design is really dragging a bit, change the bodyshell and put in some new improved internals too (iPhone 4), and when that design starts looking tired but is still saleable, put a new engine in and call it the S version as in iPhone 4S.
That's the cycle they follow to maximise profits, amortising parts manufacture over two years for products that appear to 'renew' annually. So of course the next two iPhones will have the next, new bodyshape. Probably tapered, larger screen to the edge of the phone, perhaps with liquid metal unibody (which may define the colour scheme).