I saw this kit in South Korea installed on a bike in a bikeshop. It took me about 30 seconds before the penny dropped that it was electrified. Considering I was standing right next to it, that should give you some idea of how small this is.
Çõ½ÅÀûÀÎ ¾ÆÀ̵ð¾î ±â¾÷ - HiddenPower
No good image there, but you can get the idea. A RC style motor roughly the size of a small salami, (4cm*10cm) connected to some electronics and a small battery under the top tube on the frame. The battery was roughly the same width as the frame. A handlebar cable operated clutch to engage and release the steel roller.
Downsides I could see were exposed electronic circuit boards (possibly this was an early prototype) and extra wear on the sidewall of the tire. Also the non standard battery would be tricky to replace.
I'm pretty sure that this was installed on the owners own bike.