Tod I was always under the impression that like the UK, the US allowed you to keep the old passport but you couldn't travel on it. Then, again the same as the UK, once you had the new one, the old one was completely cancelled? The UK puts a notice somehow on the system once they receive the application to stop you travelling on the old passport. It's then snipped on the corners once you have the new one.
The guy in the condo below me for instance. Offshore worker. Now needs to enter by METV obtained in the US. A good few reports on the Darkside forum of the same situation. You get away with it until they tell you no more.