To me I'd say much depends upon the nature and outcome of the game and how it affects both players and DM and what future use will be made of what was created. Sometimes to me, it is not art at all, just entertainment. Sometimes it is a sort of performance art. And on rare occasions I would say it is Art (as in, Big Art). More like the Art of a Play (which is a unique, one-time event even if the script is recorded, it will never be played exactly the same again) than Literary or Graphic Art, but if later other things arise from it such as recorded Literary Art, then it is Art more than once.
But to me Art is not simply the Product, but the Result and the Effect. And sometimes Art is recorded so that it is fixed, such as the Painting of the Last Supper, and sometimes it is unique, such as a Play by Aristophanes that has been lost to time - and just because it was lost doesn't mean it never was Art - and sometimes it lies somewhere in-between.)
For instance Craft can be both a Noun and a Verb. The craft of ballet, although I think little of ballet, is nevertheless Art if well executed. The Act was the Art. Or perhaps I should use Opera because I like some Operas. The performance is the Art. The action is the Art. Not every opera is art, but really good ones are. So being Opera does not make it Art, being good Opera does. So Crafts (verb form) can be Art as much as Paintings.
But D&D is also a game. This does not exclude it from being Art but it doesn't guarantee it will be Art either. A Musical composition is music, this doesn't exclude it from being Art, nor does it guarantee it will be Art either. They are not mutually exclusive categories, but then again they are not necessarily equivalent.
So being a Game does not exclude it from being Art, anymore than being a Man excludes one from also being an Architect. But then simply being a man does not guarantee the man will also be an architect.
That being the case then I'd say D&D at any time has the potential to be an Art. Depending on how it plays out and the effect it has and how well it is crafted. Not a guaranteed Art, as Mall pointed out, but for the same reason it cannot be excluded as an Art either.
If it is Art then it is because of Effect and Outcome as well as Product.
Sometimes music is little more than noise, sometimes it is High Art.
Sometimes a game is just a game, and sometimes a Game is High Art.
If you insist that every time a game be played it must be Art, then it is in my opinion very similar to saying that every time a film is made it must be Art. But we all know that is not true.
Sometimes the film is Art and sometimes it's a film like Dungeons and Dragons (I couldn't resist it folks).
But more seriously I'd vote yes with caveats.
D&D, or any really good game, is not necessarily an Art, but then again nothing is guaranteed Art just because it is. It becomes Art because of what it becomes. If done right.