So, not to knock DA, but to get a grip with the definitions. DA is claiming to not define art (which DA states is like defining God), yet he still imposes the idea that art requires intent. Thus DA, is in fact forming a definition, despite the fact he is denying to doing just that.
However, as pointed out by Wrecan, none of the existing definitions of art (including definitions pointed out by DA) require intent as a requirement.
So in fact, by the definitions, intent is not a requirement to create art. Art being the shared experience in the creation as it invokes an emotional response. D&D is a shared experience that invokes an emotional response.
The artist(s) in this case is not just the GM, since the players change through their interjection and participation, the final result - the creation of the D&D experience.
By these definitions, I would argue that the simplist of D&D games, even without props, music, creative dialog (nor even maps) is art. While it is not high art, it is still art.
To argue why playing Monopoly is a game and not art, all the rules and actions taken in the game are the same (based on movement, use of cards - real estate and community chest, etc) for anybody playing the game. There is no creativity required to play Monopoly. Thus playing a game does not in of itself involve art. The game Monopoly does not equal the game D&D, despite both being games. One requires no creativity, the other requires heavy doses of creative on the part of all participants. The Monopoly example by DA does not define the equality between a board game and D&D - they are not the same (not even in the same category).
We can agree that D&D is a game, but we also have to agree that playing an RPG is nothing like any other kind of game (perhaps there are some exceptions, but I cant readily think of any). So using the D&D as a game to define how D&D is not art is insufficient.
Perhaps it will never be accepted by the high artist community, but then as the wiki presents - that kind of art is only one kind and not the only kind. By definition alone (not each of our individual perceptions) D&D is art.
Since art is subjective, many of us including GMs and players might not perceive D&D is art, however this does not negate the fact that D&D is art. If a D&D session played by those who don't perceive it as art and the game session is never recorded or viewed by a non-participant - it is probably relegated to unviewed art, as the larger art community never sees it. Just because its participants does not view it as art (by their own definitons of what art is) does not negate the fact the D&D IS art.
I would argue that 'pretend' no matter if its played by a five year old or a 45 year old is in fact art - by the definitons provided.