So what are the kinds of drive systems on an electric bicycle?
http://visforvoltage.org/forum/7372-why-struggle-complicated-e-bike-kits .. talks about a GoPed designed friction drive system (http://www.goped.com/Products/GoBike/default.asp)
The drive systems I know of are: a) friction drive on front wheel, b) friction drive on rear wheel, c) hub motor in rear wheel, d) hub motor in front wheel, e) chain (or belt) drive to rear wheel, f) chain/belt to front wheel, g) trailer with its own motor pushing the bike ... any others?
And, uh, okay, if you're nutso enough you could mount a rocket engine ... but that's not gonna be electric so we'll ignore it.
The Electric Bicycle History shows patents for all those kinds of e-bicycles going back a hundred years. The earliest patent I saw was for a hub motor, in the 1890's. Friction drive systems go back to the same era.
BTW - my first electric bicycle, bought in 1995, had a friction drive. It was a ZAP DX kit. I daresay the GoPed kit is better designed than the ZAP DX kit ... One thing I've always heard about friction drives are they don't work when the wheel is wet. Something about friction not working too well when things are wet.