Dunno... there are a LOT of technical issues they will have to work out with wood. you have varying temps coming from the phone hardware causing expansion and contraction as it cools or heals up the wood. you also have moisture from the hands and its not a soft water either, sweaty secretions tend to be salty which wood reacts to in fairly unpleasant ways. Even wood boat finishes have to be renewed and maintained fairly regularly which is not something most people are willing to do on their phones. And thats not even bringing in the natural expansion/contraction wood goes through as you progress from moist summers to dry winters (remember that old hutch or drawer that closed perfectly in the winter but always stuck in the summer?).
So yea while I love making furniture and various decorations out of wood I don't really see it being a practical back material. But hey people use glass on phone backs and thats not all too practical so maybe they will try wood.