If you're an Eduspherian blogging your heart out about life in the classroom and sharing other thoughts about the world of education, then perhaps you might be interested in this resource from the Electronic Frontier Foundation: Fighting for Bloggers' Rights.
There's a handy legal guide, which while no substitute for proper advice from a practicing attorney, at least provides some background to the casual blogger. Most of the examples are not specific to the educational world, but that does not mean that they aren't worth considering. You may be most interested in looking at the material on Labor Law, which details workplace blogging. My hunch is that most of us blog about work from home and do not use hardware, software, or bandwidth available in the workplace. This is a much safer place to be. Even moreso, for those of us who work in districts with "internet nannies" (i.e. filtering software), there is even less of an opportunity for employers to stick their noses into your blogging. By not allowing blogging sites through the filter, it is not considered "school sponsored" and is therefore outside of their control. (I wonder how long it will be before a student suspended because of a MySpace post sues a school district from this angle?)
If you want to blog about your union---go for it! You've got federal protection for that. An employer is prohibited from taking any action against you as long as you're writing about union activities relating to the conditions of employment.
For those of you out there who work for private institutions, you may have less of a chance of standing up against any disciplinary measures. Considering educators as "public employees" is a bit of a stretch; however, public dollars are used for public education...and if there are things happening in your workplace that the public should be aware of---then you can make this information available. You need to be sensitive to the privacy of others; but at least edubloggers don't have to worry about divulging any proprietary information.
Finally, if you have a way to filter IPs and your employer is still attempting to monitor your blog, "this may violate provisions of federal and state laws that prohibit the unauthorized interception of communications. See 18 U.S.C. §§ 2510 et seq. (Wiretap Act), 2701 et seq. (Stored Communications Act)." While it is unlikely that anyone would take steps to sue an employer because of such a thing, it could put you in a position of strength.
Keep on bloggin'!