Thanks to Professors Bale and Secunda at Workplace Prof Blog for their post Go Away!: Novel Employee Lawsuit Against A Union and their pointer to the Wall Street Journal article In Novel Tactic, Cintas Workers Sue Unions.
It seems from the article that number of employees allege that their privacy rights under the Driver's Privacy Protection Act of 1994 "which prohibits the disclosure and use of personal information obtained through motor vehicle records, with a limited number of exceptions, including use by courts or law-enforcement agencies" had been violated. The employees suggest that the union that was trying to organize them had obtained their "home addresses from license plates in the company parking lot" in contravention of the Act.
The case is Pichler v. UNITE where the Court described the matter as follows:
"In 2003 and 2004, two labor unions allegedly obtained scores of license plate numbers from cars in the parking lots of a company whose employees they were attempting to organize. Claiming that the unions violated their federally protected privacy rights when they used the license plate numbers to get their names and addresses, the named plaintiffs filed this lawsuit, and their motion for class certification is now before us."
Albeit in a different context, you might want to look at Collective Bargaining Trumps Privacy Concerns for some Canadian discussion.