The end of live Tonight show broadcasts with Conan O'Brien has given a big boost to NBC's rivals. On CBS last week, Late Show with David Letterman attracted 4.5 million viewers, up 18 percent versus the comparable week a year ago -- and Letterman's biggest audience since last September. ABC's Nightline was up 9 percent with 4.04 million viewers. NBC, which is airing reruns of O'Brien's Tonight show averaged 2.12 million. That's not much of a lead-in for the live Late Night with Jimmy Fallon show, which saw its audience drop to 1.1 million. while CBS's Late Late Show with Craig Ferguson rose to 1.9 million.