Thanks to WikiPedia (a brilliant site, incidentally) I've found out that backmasking in songs is quite common and that the nature of the phoentic makeup of English means that it's very easy to hear anything backwards as "Satan"; this would provide an explanation for the famous Stairway To Heaven backmasking issue (playing it backwards, you are supposedly able to hear "Here's to my sweet Satan"). Nothing particularly important, of course, but I always felt that there had to be some nice logical explanation of the matter - it was a bit strange to think those wacky Zep boys would want to do something like that.