Well, iI just emailed / called our local PD. In iowa it seems nitrous IS LEGAL for street use. He said the country attorney could not find any regulatins which made nitrous illegal. The only things he said was to have it as an inhalent is illegal, and in large quantities it may be a haz mat issue.
Outside of that he said pretty much you can have it hooked up, and run it when you would like, I am sure under the assumption that you are not racing or breaking any other laws at the time.
MoSt states laws say that it is nit legal to be hooked up in a passenger vehicle. This means it is legal to have everything in the vehicle as long as the line is disconnected. So if you can drum up some "quick disconnect" you can easily be legal and still have fun.
Thank you for this update on a 10 year-old thread...