I was recently at the Toronto International Bike show and I went to the BIONX booth. They have a wheel, brake, battery combo that costs about $1000 CDN. You can set it to anywhere between 25%-200% power assist. Braking on the rear recharges the battery with a magnetic braking system, and if you crank on the rear brake, the mechanical brake kicks in. There is also a system where you can recharge by pedaling the bike normally. Their system weighs 7.6 kg (as far as I can tell).
I didn't get a chance to ride it - but it looked real cool. If anyone has any experience with them please tell me. More info at
http://www.bionx.ca.