In this hour of STATE OF THE UNION, Howard Kurtz, as always, breaks it down with his RELIABLE SOURCES. (END VIDEOTAPE) HOWARD KURTZ, HOST: David Letterman has made endless jokes about Bill Clinton and John Edwards and Mark Sanford and other philandering politicians, but it was no joke this week when he had to admit to having sex with women who worked for him at CBS. Yes, he was the target of a $