First off, Art is essentially all the stuff that humans do when they arn't trying to not be killed, or directly propagate there genes. And even those 2 things have some gray areas. Personally, I prefer to use the subjective label of, "High Art", to apply to those things which a person resonates with on an emotional level. Games can certainly be this, and often are. Adding a subjective word like, "High" emphasizes that it is a personal list, not a label you can throw around, like Ebert does.
I think its important to differentiate between games which are high art, and games which CONTAIN high art. Muramasa, for example, is filed with beautiful art. But in the same way a climactic moment in a trashy teen-sploitation flick might contain "Ode To Joy", Muramasa's art doesn't reach quite so high when you start getting down to the constant string of slashing up Ninjas. I love Muramasa, and perhaps could argue that it does reach certain levels of High Art if I wanted to, but I think that most of the High Art argument stems from the packaging, not the gameplay core. Of course, when it comes to legitimate experience, the point is academic.
Personally, I think that if you want to convert someone to believing in Video Games as high art, you need to find those games where pure gameplay is what evokes an affective response. The Marriage. Passage. Graviton. September 12th. Lose/Lose. A critic can accuse most games of simply being other forms of high art with an interactive toy attached, something adult tacked on to a childish activity. These games, there is no denying that the plays the thing.
God I feel witty for that line.
Also, I must agree with, "The Void". It is one of the rare games with truly obvious artistic depth with beautiful trappings. Once you start pluming the depths of the metaphors involved in the interaction of game play mechanics, you start putting paintings to shame.