Usually the blogger has little protection. "In most states," said Gregg M. Lemley, a St. Louis labor lawyer, "if an employer doesn't like what you're talking about, they can simply terminate you."Having opinions that aren't in step with company beliefs and sharing them with the world is risky and always has been. A blog only makes it easier for employers to find out what you think and fire you faster. As blogs become more popular companies should eventually have policies on blogs like they do with internet surfing and email. But I doubt it'll be a smooth transition. More people will be fired for their blogs before more companies wake up.
For now the best way to avoid getting fired because of your blog is to use common sense. When you're hired for a position you're not allowed to reveal company secrets and financial information, so posting that on a blog is a big no-no. Don't post any embarrassing or mean stories about coworkers; be even more careful by not posting any work stories without their approval. Don't complain about and criticize the company you work for, especially if you're new. Probably not a good idea to make racist and hateful statements either, even if those are your beliefs. With political beliefs you have more leeway because it's unpopular (and possibly illegal) for an employer to terminate workers on that basis alone. To be even safer, especially if your employer doesn't have a blog policy, never post anything about work at all.