The Terrafugia web site says they haven't gotten NHTSA's blessing to allow them to sell it as a car. Car manufacturers have to pass all kinds of crash standards, bumper height rqmts, probably EPA rules, etc. But, in a lot of states, can't an owner build or buy a vehicle and get it registered for road use? Some minor requirements (gotta have a windshield, all the required lights, seat belts, etc). So, maybe the Terrafugia could be registered as a car, just not sold as one.
Generally, yes. Laws and requirement vary by state. Here's a helpful compilation:Can you register an experimental homebuilt car??