Carbon fiber would make for a fantastic back shell IMO. It doesn't scratch nearly as easily as anodized aluminum, and even if it does scratch (depending on the finish), you probably won't notice it nearly as easily. It's also strong as hell and doesn't dent. The one draw-back is that it will snap/crack rather than distort too much, but it takes some effort to get it to that point. And, I'm pretty sure you can paint it too...which would make "Apple White" a possibility.
The problem is, IIRC, Carbon fiber is NOT radio invisible....so no gains there.
The problem is, IIRC, Carbon fiber is NOT radio invisible....so no gains there.