From the EFF:

We've just updated our Unintended Consequences report (also available as a print-friendly PDF), which collects reported cases of the anti-circumvention provisions of the DMCA being used not against pirates, but against consumers, scientists, and legitimate competitors. In the seven years since the DMCA was enacted, this has grown into quite a list:

* Blackboard Threatens Security Researchers

Billy and Virgil are both noted in the report thanks to Blackboard:

In April 2003, educational software company Blackboard Inc. used a DMCA threat to stop the presentation of research on security vulnerabilities in its products at the InterzOne II conference in Atlanta. Students Billy Hoffman and Virgil Griffith were scheduled to present their research on security flaws in the Blackboard ID card system used by university campus security systems but were blocked shortly before the talk by a cease-and-desist letter invoking the DMCA.

Blackboard obtained a temporary restraining order against the students and the conference organizers at a secret "ex parte" hearing the day before the conference began, giving the students and conference organizer no opportunity to appear in court or challenge the order before the scheduled presentation. Despite the rhetoric in its initial cease and desist letter, Blackboard's lawsuit did not mention the DMCA. The invocation in the original cease-and-desist letter, however, underscores the way the statute has been used to chill security research.