I was browsing "The Straight Dope" tonight, and came across this:

In his 1991 memoir, Breaking Barriers, journalist Carl Rowan writes that in 1964 congressman John Rooney told him that he and his congressional committee had heard J. Edgar Hoover play an audiotape of an apparent orgy held in King's Washington hotel suite. Over the sounds of a couple having intercourse in the background, according to Rooney, King could be heard saying to a man identified as Abernathy, "Come on over here, you big black motherfucker, and let me suck your dick."

A search of much of the net finds this little piece of information turning up only a few other times: notably in a Usenet message from 1990 (which was apparently written by someone with a pre-publication copy of Rowan's book). I suppose that people are afraid to say anything about Saint Martin.