If I say "art" what do you think of? Paintings? Music? Sculpture? All classic art, traditionally presented. Sometimes in quiet museums, sometimes in vibrant galleries.
How about graffiti? Fashion, light shows, landscaping, even games design. All of these have moved from the world of 'something you do' to 'something that's an artistic expression', but have yet to break into what we think of when we say "art". I believe that one day they will, even though you can't hang them on a wall or listen to them on your iPod.
Mods, however, are an interesting one. A mod, unlike a game, can be a constantly moving target. As soon as a game hits the 1.0 release then that's it. Changing too much will alienate people and ruin the original, much as the ET Anniversary Edition mucked about with the classic. With a mod sweeping changes can be made if the community want it, and can be made quickly. You can't pin a download URL for the CurrentVersion.exe in a gallery and expect people to go "ooooh".
Do you think mods are a form of artistic expression? Will they ever be seen as such by a mainstream audience? Will you one day see a demo loop from Dystopia playing at the Tate Modern?