But the unanimous ruling by the Sixth U.S. Circuit Court of Appeals says that government searches of stored e-mail violate a citizen's reasonable expectation of privacy.

NPR: Federal Court: Stored E-Mails Are Protected

But, more important will be crafting policies that will hold up under scrutiny (policies that are specific about what is appropriate or not and includes contextual examples), allow employees to use social media in ways that benefit the company, and are actually enforceable (e.g. do not violate the NLRA).

FORBES: A Primer on How to Create A Sound Social Media Policy