Do you mean a wheel-generator attachment for a manpower bike, or an attachment to borrow energy from an electric bike?

For a manpower bike, I think it would be terrifically underpowered or terrifically difficult. As mentioned in a link from Luc, http://www.ski-epic.com/amsterdam_bicycles/, even the little headlights you stick on and power with the tires make cycling _much_ harder and produce very little light.

Charging a laptop/cell off of that would be... a challenge. =P

Borrowing energy from the battery of an electric bike would be a different story, but not all that helpful. Potentially useful if you stop at a coffee shop or something on your commute, but other than that, I can't see what it would do for you.