There were loads of iPhone solar chargers at CES this year but this is one from Novothink is probably the best. It fits snugly on to your phone and can deliver thirty minutes of power for two hours of charging. It doesn't have to be connected to the iPhone to charge and if the sun isn't playing you can charge it up via USB.
It goes on sale soon in Europe including the UK, for 80 (not 8 as it says in the video) Euros. It comes with a free app that works out how much you need to use the sun to charge the battery depending on where in the world you are. See video after the turn.