The late night funnyman is retiring after more than two decades as host of CBS "Late Show With David Letterman."  Letterman, who turns 66 on April 12th, made his announcement during the taping for Thursday night's "Late Show."  Letterman's expected departure is slated for about one year from now, which will mark his 22 years as host of the "Late Show."  Citing his time hosting late night television between his eleven years at NBC's "Late Night" and his run on CBS, Letterman said it amounted to 34 years and almost six-thousand shows.