I have asked the same type of questions; the answer seems to be that it is difficult to generate enough electricity to recharge a battery and power a light. If you have interested in dynamo-powered lights, I would recommend looking around this site: http://www.pilom.com/BicycleElectron...lectronics.htm

I am currently building a small Cree 5-powered LED headlight for my son's bike (it came with a front hub generator and light, but the OEM light is not good enough). After that is finished, I will build a similar light for my commuter, using a AXA HR bottle dynamo I got from SJS in the UK.