Very, very unlikely Matt. Carbon fibre is an expensive material and to add it simply for cosmetic purposes without any structural integrity benefits would be insane. It’d make the Iphone even more expensive!
Aluminium components wrapped in carbon fibre are common place on road bikes and is a technique used to vastly reduce the amount of metal around stems and bars, giving back greater structural rigidity and longevity for less weight than all metal. And less cost than ALL carbon fibre.
The carbon fibre on the back of the Iphone is likely to be a means by which to remove as much weight as possible to counter the glass on the front, whilst keeping a piece of the phone rigid enough to stop the user bending it and protect the internals.
Carbon fibre for the sake of it on a 'standard edition' iteration of a device is highly unlikely. It'll be there for a purpose.