The laws vary greatly from state to state. Be sure to look up the particular state's D.M.V websites for updated information.
Some states require pedals, for example while others do not. Some states require a Motor Assisted Bicycles, Motor Driven Bicycles, Mopeds (depending how an individual state defines them) may require a MB only capible of 20 MPH while others do not. Even the driver's licence required from state to state differ.
Also, like Map said and my state included, the laws can be vague with a lot of confusing grey area.
So it is not as simple as where it is legal or not. The lists I have read online have mostly been left un-updated so alls I can suggest is to carfully check with what ever states you would be ridding one in. If pulled over by lawenforcement, it won't help you that some one with nothing but good intentions gave you some bad information.