Well, they don't care about the back of an iPad, so why an iPhone
To be serious, I don't think they will have any option.
As chips continue to get faster as makers try and out-do each other, they will get hotter. Android phones are being pushed and pushed as we're on the verge of 1.2Ghz dual core now, probably 2Ghz dual core in the near future and so it will continue.
Heat dissipation, like Laptops and iMac's and PC's will become a major factor in design.
You can't have large cooling vents, you can't have fans. The only way is going to be to dissipate the heat over a large metal area, and the only large area is the back of the phone, and it has to be a very conductive metal like copper/aluminium etc.
It's not going to be copper!