ComputerWorld has a very good article on the legal issues and potential liabilities facing companies whose employees blog, whether those employees do so officially or not.
The article discusses topics that include libel, disclosure of trade secrets and confidential information, careless statements that can be used during litigation, the risks of loose-cannon bloggers, and advocates corporate policies on blogging:
An employer should create policies and guidelines about what can be included on an employer-sponsored weblog. Those guidelines should describe and define the scope of the blog. They should also point out issues that shouldn’t be discussed and materials, including company documents, that shouldn’t be posted to the blog without prior authorization. An employer can also have policies or confidentiality agreements for employees who blog on their own.