Anil Dash: Nobody has ever been fired for blogging provides a good summary of the recent debate over whether or not blogging is a risk or an advantage for your career.
My own take? Assume everything you write in your blog will make it back to your boss, and that your boss will know the comment was made "in public". What would you say in other public contexts where you know your boss would hear about it?
Most companies have strict guidelines about what staff can say to the press about the company (often nothing unless cleared with PR first).
While the typical blog still have a significantly smaller readership than most established newssources, the same principle to a large extent applies - if you say something on your blog about your workplace you wouldn't dare say to a journalist or shout at the top of your lungs in public regardless of who was present to overhear it, then you should think twice.
This is no different (though the consequences may be) than giving proper regard to what you say about any other subject: If you wouldn't be prepared to take the consequences of saying something in public offline, why would you think you could evade the consequences if you say it online?