You have to think like Apple. You have to also think like a geek without going all bonkers.
They will use a technology that's relatively mature, yet innovative in their sector. It will possess characteristics of high volume, relatively low cost production and ease of manufacturing.
I personally think it will be kinetic energy. They use it in watches and it's somewhat a mature technology. However, the biggest drawback is the amount of energy it produces. Very miniscule. Using this tech will be a form of battery assistance, so regular plug-in is still required.
Think about it. When you play games on the iPhone, a lot of the games are require physics and that's where kinetic will come into play. How often do take out or play with your phone? Is your phone low on battery? Take 5 minutes and swing it around (just don't toss it) to make emergency calls.
Apple's great at re-inventing the wheel and I will put my money down on this if I had to choose.