I've had an interest in these sorts of powered alternatives (gas and electric) with the economy being crappy and gas prices going up. Would be fun to build and resell for a small profit to help people out who need cheap transportation and can't or aren't willing to do it all under their own power on a bicycle. Just the fact that ya don't have to register it as a motor vehicle and buy insurance is a huge savings in-and-of-itself.
Gas powered bicycles and high-speed electric bicycles are probably fall officially under same rules as motorscooters-- license, registration, insurance.