David Letterman can't pin this one on Les Moonves. CBS's Late Show with David Letterman has fallen farther behind NBC's Tonight Show with Jay Leno in the ratings so far this season — despite the arrival of stronger lead-ins such as CSI: Miami and Without A Trace. For the week of Sept. 23-27, Tonight Show averaged 5.9 million viewers compared to Late Show's 4.1 million.