Yet the questionable legitimacy of the "Happy Birthday To You" copyright claim is something I only recently started to hear about, first from Ben Sisto's Ignite NYC talk [2] and then again from several other copyright activist friends.
While even litigating the validity of Warner's copyright over "Happy Birthday" would be a symbolic victory for copyright activists, it likely wouldn't get very far. And let's not forget that Congress could be moved to pass a law granting "Happy Birthday" its own copyright exemption in response to the actions of a court.
[1]: http://www.wired.com/wired/archive/13.07/posts.html?pg=7 [2]: http://www.youtube.com/watch?v=mkUFlZ05EiI