Although the themes from Mario and Zelda are definitely memorable, I think it has more to do with the fact that those games were so iconic that everything about them - music, characters, voice acting, etc. - became iconic. (Also the fact that the music for both was by the same person.) I'm pretty sure that almost everyone here could hum the Elder Scrolls theme, and it'll be just as iconic ten years from now. I'd hardly call the soundtracks from Deus Ex: HR, Assassin's Creed, or Mass Effect a "lull". In fact I'd say game music is getting way better.

Wikipedia says: "Generally, art is a human activity, made with the intention of stimulating thoughts and emotions. Beyond this description, there is no general agreed-upon definition of art." Basically, anything can be art. Video games have no more and no less potential to be art than "2001: A Space Odyssey" or my kitchen table, but I don't think that the reason that they're made to appeal to the masses should stop them being art. After all, pretty much all of Michelangelo's stuff was made because he was being paid by someone to do it. I think that if something is done in the best possible way that's offered to it in its genre, it could very well be defined as art.