In the Age of TiVo and Web Video, What Is Prime Time?
Broadcast television remains the dominant medium for advertising, as the $9 billion upfront market attests, but its prime-time audience is gradually shrinking. Time-shifting has cushioned the declines, but in ways that are trickier to measure and pitch to marketers.
http://www.nytimes.com/2008/05/12/business/media/12ratings.html?ref=technology