I know this thread is a bit old but I wanted to let you know that Freedom Ebikes have now released the world's lightest 250 watt (EU max power) e-bike kit. The entire kit (hub motor, battery, controller & throttle) adds only 3.2kg to the bike.
Add it to a 11.5kg Brompton and you have the world lightest folding electric bike.

Its based on the tonxgin (Nano) super mini motor mentioned earlier in this thread but uses the latest nano-technology lithium ion batteries and a number of other weight saving improvements.
I have been riding the proto-type almost every day for the last 18 months in a lot of hilly locations.
It eats up the hills and provides amazing acceleration to cruising speed (25kph).

Check it out at: www.freedomebikes.com