Forget about freedom of speech or libel/slander laws, what she said is really unimportant because we live in a country where anyone (with enough money) can sue anyone else for anything they like.
What people use to say to a few friends is now being tweeted to a few friends. But those tweets can end up being retweeted to millions, sent around as e-mails and on Web sites across the planet. Frustrated people, blowing off some steam in a few tweets might find themselves at the end of a lawsuit. Chances are if there is no truth to what they tweeted, they are easily ignored. But if there is any truth in their negative tweets, then watch out! That knock at your door could be a lawyer.
It will be interesting to see if this Twittersuit is just the first of many to come. Of course, any company that files a Twittersuit will get some negative publicity, but that will be measured against the possible long-term damage that could/would occur to a possible endless stream of negative tweets.