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.