Actually, it's not calling for decriminalizing but something almost along the lines-- declaring it "the lowest law enforcement priority." This would put it on the same level as other non-enforced crimes such public urination, prostitution, and shootings.
The Supervisor behind it is Tom Ammiano. His reasoning is that with all the other things going on in the city, busting stoners is not what we should be worried about. He also pointed that in 1978, a measure calling for an end to marijuana arrests, Proposition W, passed with 64% of the vote. The city was all set to put the measure in effect but, well insert your own stoner joke here-- even this joke is too easy for us. Then there's the fact that this entire city seems to be made up of stoners.
What this will mean, besides strongly suggesting to cops not to arrest anyone, is that SF will refuse any money awarded by the Federal Government to crack down on dopers. An oversight committee (natch-- this city has to establish a committee for everything. We're sure somewhere out there is a Committee to look over Committees) will also be established to make sure that the non-enforcement is enforced.
So, smoke 'em if you got 'em.