I've always wondered - if one were to make disparaging comments about competitors in a blog, who would be responsible?
Much will depend on whether the company actively encourages its employees to blog (whether for marketing purposes or otherwise). If they do (or are seen to do so), usual tortious principles would seemingly apply and there would be a credible argument to the effect that comments made by an employee on such a blog falls within the scope of his or her employer's authority. Although not directly analogous, it is worth noting (not least given that the application of the law of defamation to modern technology has been steadfastly applied by reference to tradtional principles) that, unlike some American law, there is no fetter under English law in differentiating between slander and libel. As such, employers have been held liable for the rashly uttered words of their employees, notwithstanding that such words were not spoken at the employer's discretion. Thus, employers who encourage 'unsupervised' blogging are effectively leaving matters at the discretion of their employees and could arguably be considered liable for the outcome.
It would therefore be advisable that any company promoting blogging has a comprehensive policy governing use and content. Even then, however, a general prohibition on the publication of defamatory material is unlikely to suffice to exclude liability. Indeed, established case law would suggest that an employer can be liable for even unauthorised publication of allegations by an employee when that publication occurred incidentally to the performance of an authorised act. In such circumstances, the fact that the employee chose an improper method of performing his 'duty' is nothing to the point. It is not hard to see such logic applied to blogs where the employer has encouraged legitimate discussion of rival businesses.