It is a case of the Constitution versus a click. Courts have consistently found that the click wins - that employees can loose their expectation of privacy as a result of the log-in banner. In Long's case, the banner did not say that her expectations of privacy had gone poof. But where the lon-in banner does say poof, well then poof.
Compare, another recent court decision, US v Zeigler, handed down in August 2006. In this case the court stated,
Social norms suggest that employees are not entitled to privacy in the use of workplace computers, which belong to their employers and pose significant dangers in terms of diminished productivity and even employer liability. Thus, in the ordinary case, a workplace computer simply “do[es] not provide the setting for those intimate activities that the [Fourth] Amendment is intended to shelter from government interference or surveillance.” Employer monitoring is largely an assumed practice, and thus we think a disseminated computer-use policy is entirely sufficient to defeat any expectation that an employee might nonetheless harbor.To the court, it is an objective test. And if an employee clicks on a little dealie that says "I aint got no privacy," then objectively speaking, the computer that the employee is using is the employers and the employee lacks a privacy expectation in it.
Read more about the Zeigler court's discussion of an expectation of privacy in the work place, the 4th Amendment, and the particular law that protects computer network communications, the Electronic Computer Privacy Act (ECPA).