I saw someone mention 'signal loss' , but one of the more challenging issues you're going to face is "NFC loss".
I bought a rather nice cheap alluminum backed case and I lost all NFC functionality. Signal loss wasn't an issue as only the back was metal, but bye bye NFC.
With more and more merchants offering NFC payments and the like, I see such a product as this as short lived.
Now... if you can find some way to hook into the NFC antenna in the S3 and connect it to the back of your case, turning the whole back into an extended NFC antenna... now THAT would be worth having.
Just a thought to keep in mind..