When shock jock Howard Stern announced this week that he would be departing his syndicated radio show to join the lineup of Sirius Satellite Radio, it marked a seismic shift in the broadcasting landscape. Satellite is still in its infancy, and while it has been growing steadily, it has yet to attract an audience the size of Stern’s current one. And unlike other well-known hosts who have defected to satellite recently, Stern has the potential to bring a sizable chunk of the middle American radio audience with him to the other side.