I was going to say that the Nano was probably the lightest practical motor you could get, but Goat beat me to it. You won't do much better than that, some of these frame mounted motors come close when you look at how much the motor itself weighs, but they require you to *add* hardware, whereas the Nano is technically removing hardware since you're no longer counting your old hub.

BionX is the other light hub motor choice, although its not as light as the Nano it has more features, and mounts rear wheel instead of front. The 250w motor weighs about 7.5lbs as opposed to the 200w Nano's 5lbs.

Also, there are a bunch of frame mounted options out there that can weigh in pretty light. Some of them might say they have a 6lb motor, but they might also require you to pack on 2lbs of mounting equipment and an extra lb of drivetrain modification as well though. I'd look around though, something might be out there.