According to Flickr photog The Inadvertent Gardener:

As I was walking to work this morning, there was a whole line of taxicabs protesting (I think) Oakland's lack of medallions they have available to assign out. I have been able to find nary an inch of news coverage, though, so I'm just guessing...

SFist called a Friendly Cab dispatcher this morning to see what they knew. The hurried, albeit titularly amicable, dispatcher told us that "the cabs are all on strike over negotiations that failed," then she had to go right-quick.