Jeffrey Geibel has written a terrific guide to responding to web-based attacks.
Most observers, when they encounter a ‘rant' against a company or organization, will likely go to that organizations' web site to see what it says. This is a opportunity to respond and neutralize criticism. Surprisingly, most companies are clueless when it comes to this basic measure of self-defense. They simply act as if the criticisms don't exist - which is a form of denial.
The beauty of the Internet is that the gate keepers are gone. You can always put things on your web site. It won’t have the same credibility as an independent press organization, but it has the advantage or speed. Responding to negative press should begin with your corporate web site, and your employees' blogs if you are smart enough to encourage them.