It's way too easy to press it by accident on the side. You can't use the volume buttons to take phots without twisting your hand into some special shape whilst the power button is on the side. It should return to the top.
What about making the power button recessed like on Sony Xperias? If you're really worried about camera ergonomics, you can even add a dedicated shutter button. The "focus on half press" feature might be overkill for the iPhone camera system though.