How to give a negative reference

I was poking about at Mac's reading her post on being a job leper, where someone brought up in the comments that she should have a decoy call her references to make sure someone she listed isn't talking shit about her. Correct me if I'm wrong, but it's not illegal to say something that's true and can be proven (like the fact that you clocked in an hour late every day for a year and were warned about it four times before being fired), but in general, if you say something negative that prevents the job seeker from finding employment, you could be sued.

So, if you tell someone that so-and-so was bossy and had an attitude problem, they would have grounds to say that you caused them to be stigmatized by their potential employer (or some such bullshit).

I was actually thinking about giving a couple of past coworkers negative references if anyone had called, but fortunately that hasn't happened. I'm glad I've come across this article on eHow: How to Give a Negative Employee Reference. Now I realize I should keep my mouth shut seeing as how those people could be potentially litigous.