The National Bike Registry is an nationwide registry of bikes with identifying information (serial numbers, make/model, owner information) in the US that is intended to help identify stolen or impounded bikes. There are of course other bike registries such as the recent Bike Index, many municipal registries, and the International Bike Registry.
Are these registries effective, both at reducing bike theft (i.e. if you put a sticker on your bike thieves are less likely to steal it) or in tracking down stolen bikes?