Hire Rabbit has some great examples of Social Media Policies from Blue Chip corporations (IBM, Dell, Coca-Cola, Cisco, Intel).
They all have in common that they seem to follow the guiding principle “if you cannot beat them, join them.” Which practically means that they give as much freedom as possible while securing confidentiality. Positive side effect: employees who are active in social media get a certain level of security on what they are allowed to communicate so they don’t feel the need to “hide” anymore. This gives the corporation at least some intelligence about what is happening in social media. Forget about control anyway 🙂