I recently spent $23.15 to buy this:
Generator Bicycle Bike Dynamo w USB Charge for Cellphone Mobile 12V | eBay
If that link doesn't work, I'll tell you it's an ebay sale consisting of :
1. a 12v sidewall dynamo with mounting hardware
2. a short coaxial wire with bare wires at one end and a 3mm plug at the other, that connects the dynamo to
3. an aluminum box with a standard USB outlet
4. a truly frightening adapter with a standard USB plug at one end and ten different plugs at the other.
Of these, I have no use for either no. 1 or 4, but the other two are neat. No. 2 connects No. 3 to the dynamo hub.
I have not used it much, yet, so can't really comment much, but initial tests show that it will definitely charge my phone.