Strangely I was reading an article about this sort of thing in a magazine in the canteen at work today (yes we do have some strange magazines in our canteen!)
there is a lot of work being put into "energy harvesting" at the moment with the idea of turning vibrations or regular movements into electricity to power devices, one of the main commercial uses for the technology at the moment is wireless light switches, where the simple action of pushing the switch creates enough power to transmit a short radio pulse that tells a receiver/control unit to switch the light on or off. (
http://www.illumra.com/Products/Wireless_Light _Switches/index.html )
A spacehopper or a pogo stick is a fairly extreme form of regular motion so could very easily charge an iPod, but would also be an incredibly wasteful way off doing it as you would need to put a huge amount of energy into it in the first place.
One of the, possibly, most useful energy harvesting projects about at the moment for the average Joe on the street is self charging mobile phones that use radio waves in the air to charge the phones battery. now a mobile that effectively never needs charging would be useful!