depends on where you live, for sure.  IMO, quads should be totally street legal.  Think of how much gas I'd save if I rode my quad to work instead of driving one of my jeeps!!
Laws vary from state to state, and enforcement varies from town to town.  I definitley wouldn't try it in the city!!  Before I got my license I used to drive around town on my quad, pick up chix, etc....never had a problem.  But then one time a cop that I hate, and he hates me, saw me and gave me a ticket.  This was when I was in college.  Technically, it was an unregistered motor vehicle, or some BS.  If my hands and lap wasn't full of stuff, I would have just hit the gas and gone, but I had jeep parts all on my lap (400ex) and was holding a bottle of snapple in the clutch hand....
But, as I understand it, if you do register an ATV you can only like cross the street, or go down the street a hundred feet or so to another trail or something.  If you had a plate though, they could figure out who you are!!