[tmp755]

There are actually a lot of new units coming on the market soon. This article from August claims there are twice as many units "in the pipeline" as the number that made it to market in the past decade:

http://www.socketsite.com/archives/2013/08/all_48000_san_fra...

I'd guess that there are NIMBY building costs that prevented housing construction until the rent got so high, but it does look like an end is in sight.