Nice catch. It's the first imaging study to show the effect, I guess. I think this is one of the reasons for its popularity. It not only makes some folks alert. It makes them happy and alert. That's the holy combination. Caffeine doesn't quite get there and how big is that market? When we look back in fifty years at the development of custom (and cosmetic) psychotropics, I bet modafinil leads the story.
http://en.wikipedia.org/wiki/Modafinil
http://modafinil.com
Worth noting, too, that "may cause addiction" does not equal "causes addiction".