I think these guys have the right idea, I'm having one of their kits put on a recumbent right now. It's the best of both worlds; the rider and the motor have access to the gears and either, or both, can supply the power, or coast. The only downside I see is their mid-drive is limited to recumbents and one or two semi-recumbents, I'm sure if someone got creative they could make it work for a regular bike. I guess it's not cheap either, c'est la vie.
http://www.ecospeed.net/