All of the buzz Monday night had comedy fans watching and talking about Conan O'Brien, but over on the CBS television network, David Letterman had Bill Cosby on as a guest, and Cosby — though he gets sidetracked a lot easier now in his advanced age — recounted a story of a friendly competition he had with the late George Carlin. But first, a little story about a routine Cosby and Carlin both used onstage about pro athletes hawking products in TV ads. It'll all make sense somehow.