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I'm not sure how anybody would know that. The dimensions haven't leaked yet that I'm aware of. It depends on how much thinner.
 
Heard it from an Apple friend who more right than he is ever wrong, the iPad air will be like a 5 vs a 5S. Same size- just better hardware, so to speak.
 
I don't believe the rumors that they are making it thinner. I imagine those will be coming next year.

I would be willing to bet, though, that if they do make it thinner, they will design it in such a way that the old cover will no longer work. At the very least they will move the magnets around or switch the polarities in such a way that it will no longer work. Those covers are incredibly high-margin and they want to sell as many as they can.
 
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