[gambiting]

You want the wheels on a bike to spin as freely as possible, any added resistance is unwelcome because it makes cycling harder - and you would need to add resistance to generate electricity. Personally I even hate the dynamos that power the lights, because their effect can definitely be felt when you ride.

[jarek]

Modern electronics don't really need that much, and/or the dynamos have improved. I had a bike with 90s-style dynamo and it was indeed a slight pain. Today I ride Bixi/Alta bikeshare bicycles - they have something to power the lights, but any impedance it creates is too small to notice.

[greenpresident]

Hub dynamos are very efficient. I do not feel a difference riding with or without one. For street speeds of around 20km/h in an urban environment they create mechanical drag of around 6 watts [1] with the lights switched on.

http://www.bikequarterly.com/VBQgenerator.pdf

[annnnd]

I wonder if vibration would suffice? Depends on the state of the roads I guess. ;)