I am not aware of a definitive answer to your question either in the statutes or in case law, but I can set out likely parameters for making such a determination. The following excerpt from 35 USC section 102 most directly addresses the issues relevant to your question:
A person shall be entitled to a patent unless—
(a) the invention was known or used by others in this country, or patented or described in a printed publication in this or a foreign country, before the invention thereof by the applicant for patent, or
(b) the invention was patented or described in a printed publication in this or a foreign country or in public use or on sale in this country, more than one year prior to the date of the application for patent in the United States, or ...
There are other limiting factors that are pertinent to your question. The key question, already addressed earlier, is whether the YouTube video can be proven to be prior to the date of the claimed invention. I don't know enough about the innards of the YouTube system to know whether definitive proof is available, though I suspect it is - provided that the Google folks will cooperate in demonstrating the actual publication date of the video.
There is plenty of room for argument from the other side as to whether posting on YouTube is "publication" for purposes of the statute. Also, given the nature of the medium, it may be difficult to establish that the video is "truthy." Remember the videos showing fantastic basketball shots and people launched through hoops? It is also possible to misinterpret what a video actually represents, for example whether it shows an actual working process or a mockup such as stop motion animation.
It comes down to proof - convincing a court as to the factual nature of the video and the date of its publication.