We used to be able to register our bikes with the police department. They'd stamp a registration number onto the frame of the bike using lettered taps and a hammer. The only way to take off the registration is to either file it down or cover it over, in either case a clear indication the bike is stolen.
I got a bike stolen 3 times. Twice I got one back when the police found it (once the bike had upgrades too. Better front fork, better grips and 2nd rider peddels had been added)
Anyway, I'm sure someone will cite "budget cuts" but I certainly wish they still registered bikes. It seems like in a city like SF there are enough cyclists, revenues from registering the bikes could pay for a few bike police.