I would suspect that most people consider the artist-designed labels of Mouton Rothschild as examples of wine labels as art. As a lapsed art historian, I recognize the cool factor of these labels and appreciate the breadth of art styles the winery has sponsored over the years. However, I don't really consider them art. To me, they are still just wine labels, with art on them.
What labels would I consider as art? The best example, in my opinion, of a winery consistently using their labels as art pieces would be Sine Qua Non - Manfred Krankl gives each new bottling, in each vintage, a completely new name and a new label design - unique among winemakers, as far as I know. The fact that they are somewhat rough, inscrutable, and have a psychological edge to them is really something I find ultra-cool.
Know of any other wineries who, while they may not be as far-reaching as those of Sine Qua Non, are creating examples of wine labels as art?