Ok, it's big and ugly and not built into the phone, but Orange is testing this out:
Named as the "Orange Dance Charge," it's similar to the sort of strap you might use for an iPod when you're working out. A series of magnets and weights very similar to the systems used in kinetic watches is used to generate electricity.
I want to see this built into the phones, but it's a step in the right direction.
Link