A site called Plagiarism Today (!) notes a US court ruling affirming that violating the terms of an open-source, “copyleft” agreement constitutes copyright infringement, not just a contractual violation.
In effect, the court says that copyleft — a creator using copyright to set the terms by which he/she will give away his/her work — depends on the integrity of the copyright system. And copyleft gurus are cheering. Good to see this affirmation of the fundamental importance of copyright.