Ok I went to the Led Zep official site and checked around the message boards. Apparantly it is the symbol for raw sexual energy. Basically it means....F*CK!!!!
Definitely a hippy thing... I mean I would just say F*CK instead of ZoSo... Oh well they got away with putting F*CK on the cover of their album, and the industry was none the wiser !!!!! Woohoo...