“The big TV networks are on a collision course with the advertisers that subsidize their shows, including the big hits like ABC’s “Grey’s Anatomy,” Fox’s “American Idol” and NBC’s “Heroes.” The battle has the potential to get very ugly indeed over the next few weeks. And it’s all about ratings — or, more specifically, how to measure and assign monetary values to the rapidly dwindling broadcast TV audience in our era of TiVo and the Internet. Why should you, the average viewer, care?”