Answering one question and avoiding another, the Supreme Court, in a 6-2 decision, has found that although the Fair Labor Standards Act’s protections against retaliation speak of those who “file” a complaint, a wage-hour complaint need not be written in order to invoke the protective shield. Left unanswered is whether a complaint which is purely internal – presented to the employer, rather than to a regulatory body. Since the issue most worrisome to employers is that of the "squeaky wheel" employee who voices concerns within the workplace, it will be necessary to await another decision in another case to provide guidance in this regard. It should, however, also be noted that complaints of this nature often are matters of "common concern," meaning that the provisions of the National Labor Relations Act dealing with "protected concerted activity" may come into play, leaving the employer possibly vulnerable under both the FLSA and the National Labor Relations Act.
See also Saint-Gobain Ruling Leaves Uncertainty for Employers (Employment Law360)