From the San Francisco Association of Realtors:

On the evening of August 10, 2010, the SFAR MLS will begin using a new map to delineate subdistrict boundaries. All property in the MLS will automatically be reassigned that evening to their new subdistricts, where applicable. Off market data, including sold property will also be reassigned to align with the new map.

A summary of the changes made can be found here.

Very cool interactive flash map can be viewed here.