^^^The city has never done a lot of tree planting outside of parks. Most of the newly planted trees you see on the street were planted by Friends of the Urban Forest:
http://www.fuf.net/index.html , another of those volunteer groups like Market Street Railway that actually does the work the city's highly paid staff doesn't or won't do. The city does give them some money to do the planting, though, and they could always use more (if anybody wants to donate).
The problem, from my perspective, is not that a lot of trees don't get planted but that the life of a street tree in SF is, as someone once said about another subject, "nasty, brutish and short". Aside from the harsh wind, lack of rainfall from May through October and general city conditions, there are far too many vandals who seem to delight in snapping off newly planted saplings and/or pushing them over. I guess it's the way they prove their manhood.
One thing the city does do is mandate the species to plant on many streets and I think their notions are way off base. The species they seem to love (or did--recent planting suggest they are accepting reality) is the London Plane Tree or sycamore:
Source:
http://www.google.com/imgres?imgurl=...&ct=image&cd=3
That's what you see up and down Market St, ravaged by the wind and dry conditions. The city apparently likes them because they do tolerate city air pollution and soot in places like damp London, but to me they all look unhealthy in Mediterranean San Francisco (check out how much healthier they look out in the East Bay sometime). There are species that do so much better here like whatever those things are they just planted around The Infinity.