I'm skeptical that electric-powered bikes will become very popular in the US. They're fairly similar in riding qualities (lightweight, easy to handle) and licensing requirements (pretty much none) to a 50cc motorscooter, and those have failed to take off, despite being widely available in the same price range for years. I've been a day-to-day scooterist for seven years, but I don't have a lot of company out there. Especially in the north, where they're a three-season vehicle (or one-season, for the less dedicated), they aren't seen as a viable substitute for a car. Even with 100mpg engines that cost almost nothing to fuel, the ability to park them almost anywhere, and a lot of other appealing features, most consumers just don't seem interested (which is too bad for them, because unless the roads are wet or icy, I'd much rather ride than sit in a car).
An e-bike also suffers from being neither fish nor fowl. A 20mph bike is too slow to keep up with traffic in a motor-vehicle lane, but too fast to fit in with any human-powered traffic in a bicycle lane. I've ridden a 50cc scooter (mine was capable of 40mph) in 45mph zones, and believe me: motorists don't like you when you go under the speed limit in a motorized-vehicle lane. They'll eat a 20mph e-bike alive, even in a 25mph zone. But if that e-bike takes the bike lane (which isn't legal in many places), it will quickly overtake regular bicyclists, whom it won't be able to safely pass because bike lanes aren't designed for that. Dedicated lanes for motor-powered two-wheelers might help as an option for e-bikes and scooters (and motorcyclists who aren't in a hurry), but I don't see that happening until they become popular... ye olde Catch 22.