^Yeah, peter white has a light w/ that on his sight
http://www.peterwhitecycles.com/lightingsystems.htm.
It should be possible to do this on your own w/ any generator hub. The hub (6v) would act as a trickle charger to charge a battery pack with about 5 rechargeable cells. You'd have to make some electronics or something to make sure that you don't overcharge the batteries. I don't see a 12 volt hub, though, which would be usefull for charging a cell phone or other commonly chargable items.