Well, at least in one particular case. The high court ruled that supervisors may read an employee's text messages if there are suspicions that work rules were being violated. The case involves an Ontario cop who sued the chief of police and the city after discovering that thousands of his text messages had been examined. His privacy claims were upheld by the 9th Circuit but turned down by the Supreme Court in a 9-0 ruling. From the LAT:
Justice Anthony M. Kennedy said the police chief was concerned that officers might have been using their text pagers for personal messages rather than for police work. He then obtained the transcripts from Arch Wireless, the contractor for its text system, and read through them. This search was reasonable, Kennedy said, because it had "a legitimate work-related purpose" and because "it was not excessive in scope." At issue was the 4th Amendment ban on "unreasonable searches" by the government. This protection extends not only to the public but also to more than 20 million employees of state and local agencies, as well as federal workers.
The ruling doesn't fully resolve an ongoing debate involving worker privacy and an employer's right to know. "The court must proceed with care," Kennedy said, "when considering the whole concept of privacy expectations in communications made on electronic equipment."