In Borough of Duryea v. Guarnieri, the U.S. Supreme Court held that a public employee who makes a complaint to a governmental employer under the "Petition Clause" of the U.S. Constitution is only protected from retaliation where the petition involves a matter of public concern. This decision harmonizes public employee complaints under the First Amendment's "Petition Clause" with prior Supreme Court decisions involving public employee complaints under the First Amendment's "Free Speech" clause.
The First Amendment to the U.S. Constitution protects "the right of the people. . . to petition the Government for a redress of grievances," (called the "Petition Clause") as well as the people's right to "freedom of speech (called the "Speech Clause)." This case concerned the extent to which public employees are protected by the Petition Clause when they make routine complaints to governmental employers.
Guarnieri was a police chief for a local borough in Pennsylvania, who filed a union grievance against the borough, challenging his termination. Following a subsequent arbitration, Guarnieri was ordered to be reinstated by the borough. After his reinstatement, borough council issued 11 written directives to Guarnieri concerning the performance of his duties. Guarnieri then filed a lawsuit against the borough, arguing that his original union grievance was a "petition" that was protected by the First Amendment's Petition Clause, and that the 11 directives that were subsequently issued by borough council were issued illegally in retaliation for Guarnieri's protected activity in filing a petition.
Diverging from the decisions of other Circuits, the Third Circuit Court of Appeals agreed, and held that Guarnieri's petition (in the form of his union grievance), was protected under the Petition Clause, even if the content of that petition did not address a matter of public concern.
The U.S. Supreme Court disagreed, and reversed the decision of the Third Circuit. In a 7-2 decision, with Justice Thomas filing a concurring opinion and Justice Scalia filing an opinion concurring in part and dissenting in part, the Court held that in order to find protection for complaints to governmental employers filed under the Petition Clause, public employees must be petitioning about a matter of public concern. The majority opinion noted that public employees who complain to their governmental employers enjoy protection from retaliation under the First Amendment's Speech Clause only where their complaints involve "matters of public concern," as opposed to matters of "purely private concern." Given this premise, the majority found no distinguishing reason to treat a public employee's "petition" to a government employer under one section of the First Amendment differently from a public employee's "speech" under a different section of the First Amendment.
Therefore, the Court's majority laid down the following rule: "If a public employee petitions as an employee on a matter of purely private concern, the employee's First Amendment interest must give way, as it does in speech cases. When a public employee petitions as a citizen on a matter of public concern, the employee's First Amendment interest must be balanced against the countervailing interest of the government in the effective and efficient management of its internal affairs. If that balance favors the public employee, the employee's First Amendment claim will be sustained. If the interference with the government's operations is such that the balance favors the employer, the employee's First Amendment claim will fail even though the petition is on a matter of public concern."
You can read the Supreme Court's full decision in Borough of Duryea v. Guarnieri here: http://www.supremecourt.gov/opinions/10pdf/09-1476.pdf