Republican presidential nominee John McCain's most relentless detractor in recent weeks may not have been Democratic contender Barack Obama, but CBS's Late Show host David Letterman, who has continued to fling zingers at McCain for canceling an appearance on his show at the last minute on September 24, then turning up at the appointed time for an interview with Katie Couric just a few blocks away from Letterman's studio. McCain, who formally announced his candidacy on the Late Show on February 28, 2007 has been booked for Thursday's edition of the show, CBS said in a statement.