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I look at the iPhone 5 speculation with this thought it mind: If the iPhone 4 had been presented and released with the bumper as an integral part of the iPhone design, the PR headache of AntennaGate, GlassGate, GripGate, and SteelBandScratchGate would have never happened.
So I speculate that the iPhone 5 will likely be an iPhone 4 with a few bumped specs (like 64GB), a few new little features (like NFC), and a perma-bumper (slightly redesigned).
Personally, I'd like to see all that blown up to a 4" screen, but I doubt that'll happen.