Edit: The following link seems to match many of the attributes mentioned in the complaint filed (date of talk, sponsor of talk, his keynote speaker status, location of talk, video length).
http://youtu.be/KBTWoCaNKn4?t=37m41s
Edit2: Confirmed. This is the talk. The Ars article says the title of the talk was "Open," same as the title in the video. The "infringement" starts around 37:40. Funnily enough, the "infringed work" is apparently a remix itself. Go figure. You couldn't make this stuff up.
Edit3: I can't tell whether the song itself is a remix, or if Lessig meant there were several remixes of the original clip that had the music dubbed over clips from The Breakfast Club. It appears to be the latter.
Suffice it to say, this is fucking ridiculous. Raise your hand if you're going to watch a 49 minute long video in order to try and "home tape" the 47 second clip 37 minutes and 40 seconds in, especially if you can just download the official video from here:
https://www.youtube.com/watch?v=4BJDNw7o6so
Whether this is "official" as in this is the actual video shot for the song, or "official" as in "welovephoenix" is their official youtube channel (or both), I don't know. But that link has nearly 10 million views.
Edit4:
And this is the original video remixing the song with The Breakfast Club and another 80s movie I'm too twentysomething to recognize, sitting pretty at 458k views: