According to a recent article courtesy of the New York Times it is.

However, the criticism is restricted and needs to be reasonable as the follow section of the article suggests:

...Labor law specialists say employees have the right to criticize or disparage their companies or supervisors as part of a conversation aimed at improving working conditions, but do not have the right to merely curse supervisors or make untrue, disloyal statements that damage a company’s reputation...

For more details see Labor Panel to Press Reuters Over Reaction to Twitter Post by Steven Greenhouse.

Thanks to Union Renewal blog for story.