There were a number of such attempts before. For national WiFi ISP's the most expensive part is the airway license cost. This cost often kills the project. Other community/local governments' attempts at providing free Wi-Fi ended with the skyrocketing cost of expansion and maintenance.
My opinion is that Free WiFi is possible in a very small area. Over entire S.F.? Only if Google is capable of pumping a lot of money into it without much of a hope to make any of it back.
