It's probably already decided that the other half will be a kind of backcover but if it was my decision, I would really split the phone in half, and put the camera and battery in the back. This would allow more upgradeability and really different configurations for different usecases. Want a cheap slim phone? Buy it with a 5mpix shooter and a small battery. Want a camera; get the 12mp protruding back with tripod mount. Back cover may also hold extra emmc.

It has it's drawbacks that backcovers won't be cheap, so getting more of them just to change colors won't be easy, but we have cases for that