http://en.wikipedia.org/wiki/Backward_message
"Backmasking first became famous with The Beatles. Just before the band's break-up in 1970, DJ Russell Gibb initiated the infamous "Paul Is Dead" urban legend (a rumor that Beatle Paul McCartney had died) by playing certain Beatles records backwards to reveal hidden messages. One album in particular, The Beatles (aka The White Album) was said to contain backwards messages. Intentional gibberish at the end of "I'm So Tired" supposedly is "Paul is dead, man, miss him, miss him..." Likewise, the repeated words "Number nine, number nine, number nine..." in "Revolution 9" are supposedly "turn me on, dead man, turn me on, dead man..." backwards."