When Apple introduced the iPhone, it shook up the cell phone business, but it also changed the way people play video games. About a quarter of the 100,000 applications that can be downloaded to the iPhone are games, ranging from the simple "Doodle Jump" to a version of "Grand Theft Auto." Now, with the looming launch of a tablet-style computer — already nicknamed the "iPhone on steroids," with a bigger screen that might have a place in the living room — Apple could pose an even tougher challenge to established players in the video game business. That assumes, however, that potential buy...