Well, it's right here. The developer calls its Transition vehicle, which just had its initial test flight, a "roadable aircraft." The prototype is licensed as an aircraft. What it can't do, legally, is pull a Chitty Chitty Bang Bang and take off from a public road to escape a traffic jam. However, if these get popular (they should go on sale in 2011 for under $200K), you know someone will try it.