Bah, all of this stuff is old hat.
I have a hilarious book on the subject called "The Devil's Disciples" written by a born again christian on a crusade to end the so called evil influences of rock n roll, specifically metal.
As a wee lad, I found the themes fascinating yet the pontificating christian parts amused me greatly.
There's a whole chapter dedicated to all kinds of backmasking messages in it. The Beatles' most famous example would be from the White LP's Revolution 9. "Turn me on Dead man", all a part of the whole 'Paul is dead' joke.
Regarding Led Zeppelin - if you're a fan and are skeptical about the satanic influence, it should be duly noted that Jimmy Page was highly influenced by Aleister Crowley, one of the most infamous of satanists.
Its all rather amusing though....
As Slayer might say: Satan.Laughs.As.You.Eternally.Rot. [img]graemlins/hehe.gif[/img]