This is nearly 100% sure to not happen, but food for thought:
Wouldn't it be interesting if Apple re-released the 4s with this new housing as the lower-cost model, and then introduced the new iteration of iPhone with the same size screen but a new housing? Then it would be "innovative" design for new phones as everyone is screaming for, but all new phones being sold (assuming the drop the 4 and 3gs) would have the same size screen.
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Let's face it - the hardware probably won't see a refresh again until 2014, which means their single telephone product will have seen only minor tweaks in four years.
So you're definition of "hardware" is simply "casing"? Because that's just silly. If the iPhone 4s would have been inside a different housing, would it have been a "major hardware refresh" even if everything inside the phone stayed the same?
Who cares if the internals get a new house - the iPhone 4 design is attractive and functional.