The Industrial Workers of the World (AKA IWW or Wobblies) have been trying to organize Starbucks for much of the past decade and many baristas have become dues-paying members of the union as a result of these efforts. At least one barista was coming to work wearing several pro-IWW buttons. Starbucks argued that by wearing more than one button they were converting themselves into “personal message boards” for the union. The 2nd U.S. Circuit Court of Appeals recently ruled in Starbucks’ favor, according to MSNBC.
The court ruling overturned an earlier National Labor Relations Board decision that allowed the wearing of multiple buttons as free speech, saying the "board has gone too far in invalidating Starbucks's one-button limitation."
Very few (if any) of the thousands of Starbucks outlets are currently unionized.