If you're wondering how many cities will eventually grapple with the legal issues surrounding Airbnb and other internet-based home rentals, you may not have to look much further than San Francisco. Its Board of Supervisors has passed a law that lets these services run, but regulates them to both prevent abuse and give the municipality its due. Hosts not only have to limit whole-home rentals to 90 days per year, but register with the city's Planning Department (which costs $50), pay hotel tax and keep records that prove they're respecting the time restriction. There should also be companion legislation in the future that prevents landlords from evicting tenants to turn their buildings into makeshift hotels. If implemented, it'll let housing non-profits sue to stop these turnovers rather than wait for officials to respond.