Apple has been granted a patent related to a solution that allow portable devices such as mobile phones and portable media player to use solar power to supplement rather than replace their onboard batteries.
Apple's solution describes a voltage converter coupled with a controller and a solar power source that allows the sun to charge the onboard battery. It may well find its way on the next iPhone or the next iPad either as the back cover of the device or a "green" power source.
Rather than direct sunlight, most solar panels can work perfectly well with ambient light although any solar panel would be useless if the phone is left in the user's backpocket most of the time.
Charging a phone however means that you will need to put it either on a flat surface or in a docking station for a few hours, which should prove enough for a few hours.
Should you require something more reliable, you can always try hand-wound mobile phone chargers like the LED-99CHR which will give you some precious talk time.