The moral of Employers Need To Be Careful To Avoid Waiving The Protections Of Written Computer And Email Policies from The Labor & Employment Blog is that you had better:
The plaintiffs sued both Arch Wireless and The City of Ontario, as well as some individual defendants. In pertinent part, the Court held that in light of the lieutenant's informal policy that he would not audit a pager if the user paid the overage charges, the plaintiffs had a reasonable expectation of privacy in their text messages as a matter of law – notwithstanding the City's written policies to the contrary.
This case cautions employers that it is possible to lose the benefits of their written computer and email policies. Employers wishing to maintain their rights should ensure that managers and supervisors do not inadvertently undermine the policies by adopting inconsistent practices or making inconsistent promises to employees.
For those without these kind of policies should also read the full post. I think the post gives a good idea of how such a policy should read.