Maybe not directly relevant, but in the case of Waves Audio Ltd. v. Skyline Recording Studios NYC, the jury ruled that "[a] person is liable for copyright infringement by another if the person has a financial interest and the right and ability to supervise the infringing activity, whether or not the person knew of the infringement" [1]. Although this is a very different case (cracked software was used to mix music), I wonder if something of a sort can be applied to Google here—they have the right and ability to "supervise the infringing activity" i.e. the Content ID system gives them the ability to know precisely if an infringement occurs, so a DMCA notice that is not compiled with might not be necessary for this to go to court. I'm not sure how it would hold, though, this is just a thought.

[1]: http://www.prosoundweb.com/article/waves_audio_wins_case_aga...