Twitter, in a blog post published today:

This morning, we filed suit in federal court in San Francisco against five of the most aggressive tool providers and spammers. With this suit, we’re going straight to the source. By shutting down tool providers, we will prevent other spammers from having these services at their disposal. Further, we hope the suit acts as a deterrent to other spammers, demonstrating the strength of our commitment to keep them off Twitter.
To the death!