What Frank Petronio says. The photographs are documents. The art is the event in its totality as a performance ritual. So it is conceptual art - art based on an overarching concept, rather than art based on aesthetic expression. As a concept the work is transgressive in terms of common social rules of decency and privacy, it denies the substantial power of government and law in an uber-passive way (naked, lying down - hard to get more vulnerable than that,) and it challenges individually held &/or repressed aspects of identity that have to do with the body and how it is displayed/withheld in the public arena.
So if you evaluate the photographs only as art, you'll be missing a significant part of what makes the complete work. And as Nikon Sam hints, the most complete way to experience the work of art is to participate in it. Taking your clothes off in public in the midst of a large number of other naked people is an action diametrically oppossed to what most people have engrained into their being by their self-preservative instincts. So the work challenges the individual and how the individual views themselves within the group.
Art does that.
Now the more interesting conversation might be whether it is good or bad art, or if it is effective in achieving what it projects as its goals, but debating whether something is or is not art in the twenty-first century is philosophically equivalent to debating whether the moon is made of cheese. Art, just like everything else in this world, has been so infinitely commodified in this world, that the debate as to its limits and extremities is nearly meaningless.
