Feasible for lightly charging or maybe at least "stalling" typical battery usage? Maybe ... I would hope for Witricity wifi charging first!
Also, as others said, it's not good to encourage users to put their phone in the sun as the heat is bad for the phone. Plus, I spend most of my time either out at night, in the office during the day, or in my car. It doesn't leave much opportunity for solar power.
It wouldn't do much good as the phone is usually either going to be in your pocket or most of the back will be covered by your hand both of which will keep it from charging. Also the phone is too small to produce that much electricity even if it was sitting in the sun.