Bicycles are frequently fitted with dynamos or generators to power lighting at night. All you need is a suitably small and lightweight voltage regulator to take the generator output and convert it to 5 V DC for device charging. Then you just dock your device on the handlebars and charge it while you ride. Simples!
(This is especially useful if you use your device to track time, distance and location during your ride.)
That's not a bad idea, however I was more on about the practicality of a general charger for the phone, than one specific for my needs
More practical for bus riders are the boots that steal some of the compression from each footstep. They can actually generate a fair amount of power, though the more power generated the more it is like walking in deep sand or mud. The power captured by watches is not nearly enough for cell phone power.
The boots thing sounds interesting, and your right on the small amount of power made by watches, but a scaled up version? I was thinking more of a trickle charge type thing, that may only put a tiny amount into the battery over time could keep it at a reasonable level, or at least extend the life of the battery for a short period