I have read, since this whole Letterman story broke, that Jimmy Kimmel is dating one of his writers. Carson Daly, actually impregnated one of his staffers and Conan O'Brien married a woman who was an actress/skit player on his show.
It seems as if Letterman's behavior is standard operating procedure in the late night biz - and probably all of Hollywood.
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There, fixed it for you. Hollywood is its own (im)moral enclave.