depends on where in san fran you're referring to ... anywhere close to the bay will get *very* cold and windy. a winter jacket is sometimes needed.
but if you're talking about the core or in the greater bay area it is generally nice during the day (light clothes, possibly t-shirts most days) ... at night it can dip down to close to freezing but normally not under.
i was just there in december and it was nice for the most part, a fall jacket was all i needed for the week at night.
--craig