Considering a lot of people around here don't have cars and take transit, the bumper sticker is not an available option of displaying your politics or causes. However, I've noticed even before the presidential election that messenger bags can have the same affect. After the election started though, they have increased exponentially around here. Mostly Obama and No on Prop 8. I've noticed people on the train checking out my bag as well, which has my own "urban bumper stickers"
I've got the longhorn patch which has brought me closer to fellow Texans randomly on BART and the Hey Mercedes buttons of my favorite band that have gotten comments from some rock kids. Though no one has said much about the SF Municipal Railway or the Market Street Railway pins, it might be because we're usually smooshed together on the J Church.
The thing though is that unlike bumper stickers, you have to be in close quarters to see what the pins and buttons say, which makes people a bit more cordial than if they were in the space of their own car. No honking on transit.