Maybe prime time isn't prime anymore. The post-dinner, pre-late-news stretch of the networks' offering may no longer be TV's most valuable time slot. Since the new season began, the top five broadcast networks' average audiences are down in the high single digits. Yet the networks' late-night shows, from "Late Night with Conan O'Brien" to "Nightline" are up around 5%. Morning shows also are holding their viewers much better than those in the evening. The numbers further indicate that more people now watch early-morning and late-night TV than 10 years ago, per Brad Adgate, Horizon Media senior vice president, …