I think ever since visualization got started, people have been asking this question.
Some... okay, many describe it as purely an analytical tool. Others (i.e. me) are a little more liberal with their use of the term while the rest are somewhere in between. Some insist that the stuff we see on information aesthetics belong in an entirely different category and that that stuff isn't visualization at all.
As art, science, design, statistics, computer science, etc. start to melt together, the line between what is and isn't visualization grows more blurry.
What do you think? Is visualization only analytical? Can visualization be art? Are the infographics that frequent the front page of Digg visualization or are they just pretty pictures? Can visualization be just a pretty picture? Leave your thoughts in the comments below.