The meaning of an inverted cross

"If those Satanists had paid attention in Sunday school, they would probably realize that the inverted cross is actually the personal trademark of Saint Peter, the first Pope, and one of the most revered figures in Catholic lore. When Peter was martyred by crucifixion he was said to have requested to be crucified upside down because he didn't feel worthy of dying the same way as Jesus. As a result, many dyed-in-the-wool Catholics actually consider the inverted cross to be a more acceptable thing to attach to your tacky jewelry than a regular right-way-up one... By wearing an upside-down cross, Satanists are unwittingly showing humility and unworthiness before Christ.

More details at Cracked.  Wikipedia points out the essential difference between an inverted cross and an inverted crucifix:

In Roman Catholicism the Petrine Cross is not seen as Satanic in any way. However, an inverted crucifix (a Latin cross with an artistic depiction of the crucified body of Christ upon it) is seen as immensely disrespectful, and could be used to represent Satanic forces. The distinction between a Cross of Peter and upturned Crucifix is sometimes obscured, leading to confusion about the acceptability of each symbol. This was seen when controversy arose over the aforementioned Papal visit to Israel; pictures of the Pope sitting before a Petrine Cross were widely circulated on the Internet in an attempt to prove that the Catholic Church is associated with Satanism and the Antichrist.

Via Cynic**:.)