Like someone else said, it's legal here in AZ. Hell, I see people riding them just about every day around here. They get plated as a motorcycle. That said, I'd still take a motorcycle over a quad. Can't wait to get back on 2 wheels (hopefully by this Spring!).
The side-by-side quads can be made street legal here, too, but they're not as common on the roads as the quads (I've seen maybe 2 or 3). All they have to have are headlights, brake lights and indicators to get plates.