This is a question that I get quite often from employees: What are the limits of what I can say in the workplace? The answer is usually "Not as much as you think." Many employees are surprised to learn that, generally speaking, employees of private sector employers have no constitutional "free speech" rights in the workplace.
The Delaware News Journal has an article on the limits of free speech in the workplace that employees would do well to read if they have questions in this are.
We have previously covered instances in which employees have been fired for posting something on their blog that their employer did appreciate. With the advent of Facebook, MySpace and other social media, the problem is only becoming more acute.
Employers are wisely beginning to respond to the situation by publishing their own policies governing such communications. You can find IBM's blogging policy here. And here is Yahoo's policy as to personal blogs.
If your company does not have its own policy yet, you might give these two policies a look to get a general idea as to what an employer's expectations will likely be regarding its employee's conduct in the online world.