I also registered my bike on the National Bike Registry. When I sold my bike at a local bike consignment shop they mentioned that many people don't keep their serial numbers in an easily accessible place. So when their bikes get stolen they can't find where they kept their serial number written down.
In Portland, OR bike consignment shops have to keep your bike for two weeks and check it against local police dept. Reports of stolen bikes, so as not to be selling stolen bikes.