Solar power is wonderful. Really, it is. And if you don't think so, it means that you hate the planet, and you don't hate the planet, do you? The one teeny tiny little problem with solar power is that so far it's proven to be more or less useless with the things that could really benefit from it, like mobile electronics. So what's the problem?
Nokia says that the problem with powering a cellphone using nothing but the sun really comes down to two things: first, phones suck down a lot of power. Even on standby, with the screen off, you're looking at about 10,000 times as much power draw as one of those solar powered watches. This doesn't just mean that you need more solar power overall, it also means that you have to be very aware of how you're using your phone. You'd need to leave a solar powered phone out in the sun as much as possible (not in your pocket), and you'd need to use it minimally. Most consumers probably aren't willing to make compromises like that.
The second issue is that solar panels are expensive, especially compared to other kinds of chargers. People who are willing to pay a premium for a cellphone generally aren't the same people who are willing to deal with a compromise on usability. And of course, the more affordable panel is, the less power you get out of it, and nobody wants to go too far in that direction.
For their part, Nokia recognizes that despite these shortcomings, solar powered phones could be important tools for the developing world, so they've embarked on a three month test project to try and see what it would take to develop a (relatively) cheap and effective cellphone that relies primarily on the sun for power. Their first stab at a prototype is the "Lokki," pictured above, which actually uses a stick-on solar panel combined with a data logger. After the project is up, Nokia hopes to take what they've learned and figure out how to make solar powered phones a reality.
You can follow along with Nokia's project, which includes beta testers in Sweden, Kenya, the Arctic, and the middle of the Baltic Sea, at the link below.