I was doing a one-way rental from BOS to Northern NH (on the ME line) one time. When I went to pick up the car from Enterprise, they said "oh, we have a certain one we want you to take." When I stopped off to get something to eat a little later I realized it had expired ME plates and inspection on it. Fortunately I didn't get pulled over, but I'm curious what would have happened if I was.
Also, why aren't rental car companies able to register as a fleet in one state? I've noticed that moving trucks do. All U-hauls are registered in AZ, Penske rental trucks are tagged in IN, and Budget trucks have OK tags. This would likely make things much easier if it were possible.