The results from Nokia's solar charging experiment are in, and while it is possible to juice up your phone via the Sun's rays, it's nowhere near consistent enough yet.
For the experiment, Nokia have used their C1-02 (one of their millions of dumbphones), attached solar panels onto the back, and sent them to a variety of countries over the last six months. Five participants were selected from across the world: two in the Arctic Circle, one in Southern Sweden, one in Kenya, and one in the Baltic Sea, each with varying lifestyles and varying weather conditions to give this a fair shot.
The results that were achieved were pretty common sensual: the device situated in Kenya harvested the most amount of sunlight and gained the best charging results, due to the climate there. However, as was most probably expected, the Arctic pulled the least. It even required the test subject (a builder in the Arctic Circle) to move his phone from one side of the house to the other to gain some charge. Other particular statistics showed that more solar power was gained when the phone was worn around the neck; but then again it doesn't really look good, and it's not particularly safe (burglar proof).
So the main conclusion is pretty clear. Solar panel charging is possible for mobiles; but it's not yet ready for primetime.
Source: Nokia Conversations