The lesson from Spiderman, with great power comes great responsibility.
The great thing about blogs is that it allows anyone to be a publisher. Most people don't think of themselves as journalists, but in a lot of ways that's what they are. While they usually develop a community and typically have a specific target audience in mind, blogs are readable by anyone and words expressed today are still accessible weeks, months and years from now. Hopefully these stories will make bloggers think about what they're saying before they publish. Not that they shouldn't express themselves or anything like that, but heat of the moment words on a blog have a permanence that a rant to your spouse does not.
Things to keep in mind are any Non Disclosure Agreements or any kind of codes of conduct you signed. Of course it's much more clear with an NDA, codes of conduct are often signed when you first start working for a company. So many people don't actually read them and are sometimes even discouraged from reading them fully, and employers change or add to them along the way (which seems pretty shady to me.)
Of course the First Amendment still applies--say whatever you want and no one can tell you otherwise, and aside from NDA or code of conduct which you have agreed to, your company can't tell you how to live your life when you're on your time. However understand that your employer may not be so happy with you if you constantly slam the company. Whether dismissal is legal or not depends on the situation, just keep in mind that what you say in a blog (or on a forum like this for that matter) is of public record. Balance your desire to say borderline things with your desire to keep working with your current employer.