It appears that there is not a single case decided after the Trademark Revision Dilution Act (TRDA) in 2006, in which there has been a finding of trademark dilution not accompanied by a finding of trademark infringement. This cause of action had ceased to exist after the Supreme Court’s 2003 decision in Mosley v. Victoria’s Secret, which interpreted the 1996 Federal Trademark Dilution Act.
I'm not entirely convinced by this data, as three years is not a lot of time to get a case with the right facts before a court. It matters because trademark dilution by tarnishment is theoretically, at least, much more threatening to free speech than a mere trademark infringement, which is really just a subtle species of fraud (i.e. attempting to get a consumer to think that someone else made the goods or services marked). Credible legal threats to free speech can do harm, even if the matter never gets beyond the cease and desist letter stage, and hence doesn't produce any public court decisions on the matter.