Although a 16% increase in the San Francisco for-sale listings is hardly substantial, it is directional (and important to note). According to the New York Times:

For-sale listings have also swelled throughout California, according to the California Association of Realtors. In the San Francisco Bay area, they have increased 16 percent in the last year, Coldwell Banker Residential Brokerage said…

“We are seeing a market in transition,” Leslie Appleton-Young, the association’s chief economist, said.
Brokers said that some houses seemed to be on the market longer because sellers priced them too high, assuming that their value was still rising sharply. In other cases, people who otherwise would have waited a year or two to sell their homes – like empty nesters ready to move into smaller quarters – had listed them now out of fear that prices would soon fall.
Some economists and commentators have for years predicted the bursting of a real estate bubble, and previous slowdowns have turned out to be relatively brief pauses before prices started accelerating again.
But with mortgage rates now rising, the cost of gasoline hovering at or near $3 a gallon and house prices in some areas out of reach for many families, brokers and analysts said they thought that this slowdown could be the real thing.

Again, even with the 16% increase in inventories, the San Francisco market remains a “sellers” market (based on historic metrics). But if it’s a trend, and new developments continue to come online in SOMA, Mission Bay, and Bayview, then watch out. The tide could quickly turn.
Slowing Is Seen in Housing Prices in Hot Markets [NYT]