I've seen 3 bumper stickers in Pittsburgh for Ron Paul, none for any other Republican candidate, and three Obama stickers.
One of the Ron Paul Stickers was outisde Trader Joe's: "I <3 Ron Paul." another was on a car of a guy who also supported the local Green Party Candidate (Titus North), and another one was on a guy into some kind of repair work. He also had an End the Fed Sticker and End Corporate Bailouts sticker. In my sampling, Ron Paul dominates decided bumpers.