Now this is one of those great questions. Mainly as the rule of thumb for most bloggers is to avoid this at all costs. However, according to the authors of Blog, Blogger and the Firm: An Analysis of Firm Policies says
Paradoxically, under certain conditions negative postings by employees can actually help the overall reputation of the firm. The rationale for this is that negative posts raise the credibility of an employee blog and attract more readers, who then will also be exposed to the positive posts on the blog.
Now this is obviously true, and if you’re a blogger such as Robert Scobble, when he was at Microsoft, I would say it has also been proven to be true. However, I don’t think if I adopted this approach about my employers it would go down too well!
A really good read though.
[Thanks to Ideoblog for the pointing me towards this article]