Ebert discusses some of the various definitions and features of both art and games, and that's really where the whole issue lies. "Art" and "game" are very abstract, poorly-constrained concepts that often overlap very different conceptual space for different people. The issue probably just boils down to how liberal your definition of "art" is. Mine tends to be fairly liberal. I'd probably define art something along the lines of: The arrangement of elements by one or more agents in order to provoke thought and/or arouse emotion.
I think agency is important to the definition, because though accidental arrangements can often be beautiful, the very idea of art seems bound up in the notion of intent, and there is no intent to arouse awe in a volcano or provoke self-reflection in a sunset. And I think noting that some art is intended to make you think, while other art is intended to make you feel (and often great art does both) is important to a definition as well.
Ebert mostly seems to object to the idea of games (and not just video games) as art because you can win them, and because they have rules, points, objectives, and an outcome. But this is a weird objection. Every art form has elements that distinguish it from others.
One obvious difference between art and games is that you can win a game. It has rules, points, objectives, and an outcome.
But we could play all day with definitions, and find exceptions to every one. For example, I tend to think of art as usually the creation of one artist. Yet a cathedral is the work of many, and is it not art? One could think of it as countless individual works of art unified by a common purpose. Is not a tribal dance an artwork, yet the collaboration of a community?
One could make the same objection about any art form that Ebert clearly considers true art:
One obvious different between a cathedral and art is that you can house hundreds/thousands of people in a cathedral. Cathedrals have doors, windows, spires, and holy people.
Try it with any form of art that Ebert clearly recognizes as true art: dancing, theater, literature, and of course film. You can clearly find stark differences between them that seemingly set them apart from the others. Some have clear function outside of their aesthetic appeal. Some are artifacts, while others are temporary performance. The thing to do, in order to have a consistent concept of what something means to be art is to identify the common features, not the differences.
And I tend to think that common core is a desire to evoke emotion and/or provoke thought by attempting to put things together in a way that nobody before has done. That to me is the essence of art.
So, under these fairly broad guidelines, of course video games are art.
Another issue is the distinction between good and bad art. Ebert skirts around it, but never really comes right out and says what he thinks on this point. My guess is that he considers even very bad films art (albeit bad art), while even the most gut-wrenching, thought-provoking video games are not art.
And I'm sorry, I just can't abide any opinion that considers Ernest Goes to Jail art, but doesn't acknowledge Myst as art.
Oh, and P.S. Learn how to fucking use hyperlinks, Roger. It's 2010.