many of these attach to packs, but the reports I have seen say that angle to the sun is extremely important. Anything even slightly off nominal and you get zero charging (and for a poorly designed system you would get battery discharging!).

Solar cells are getting better all the time... and people have high interest in them considering how often this issue get rehashed on here and bpl. There is a really spiffy DIY project on bpl designed to use a solar cell to directly charge your iphone in a quite lightweight package. It is a very interesting (to a tech geek like me) and very technical discussion on circuitry and the challenges of solar panels.