I have looked into this in California a little bit. The confusion begins with Section 406, the definition of Motorized Bicycle; section 405, Motor Driven Cycle; and section 400, Motorcycle:
http://www.dmv.ca.gov/pubs/vctop/d12/vc24016.htm
http://www.dmv.ca.gov/pubs/vctop/d01/vc405.htm
http://www.dmv.ca.gov/pubs/vctop/d01/vc400.htm
As I see it, 406(b) covers many motorized bikes with pedal power. 406(a) lets you go up to 30 mph without cranks. Section 405 is not of much use. To go over 30 but less than 45 mph you are a 400(c) motorcycle.
I have mailed in a form Reg 230 to "register" my electric bike as 406(a). I could not see anywhere that told me the price, but Thunderstruck told me it was $17 last they knew, so that is what I sent in. It is a 1 time, lifetime, transferable registration.
http://www.dmv.ca.gov/forms/reg/reg230.htm
I am not sure what I will get, maybe a sticker.
I plan to eventually classify my bike as 400(c) motorcycle, so it needs turn signals, rear view mirrors, brake lights, a horn, and head and tail lights. I think. There are also laws about how the headlights have to turn off with the keys and dozens of others. They are peppered through the DMV code, and it is not always clear what class of vehicle they apply to.
To register and title a vehicle with no PIN in California, I will have to go through Registration Requirements for Home-Made, Specially Constructed, or Kit Vehicles:
http://www.dmv.ca.gov/vr/spcnsreg.htm
Yikes.
One problem with this is that is pretty fuzzy. Most peace officers do not know what 406(a&b) and 400(c) are. I called the CHP here and they insist that any bike with a motor of any kind is illegal on the road, not registrable, and they told me that they had impounded many. I hope there was a lot of miscommunication. The CHP has to inspect and approve any kit vehicle in order to register it. I like the idea of calling a local chopper club. Maybe some CHP stations are friendlier or more knowledgeable than others.
There is irony here. Green environmental groups like the Sierra Club have a clear goal of making car ownership more difficult through increased regulation. Yes, most laws are there for safety. But these laws are now making it more difficult to innovate new kinds of transportation: not what environmentalists intended, I am sure. Eventually I think the vehicle code has to change to be made more explicit and permit other kinds of electric vehicles. And peace officers have to be more aware of the laws and the various kinds of vehicles that are permitted under the law. And we will have to start registering these vehicles and paying taxes and fees on them.