According to 'Encyclopedia of World Mythology' (ed. by Arthur Cotterell), the castration rites stem from a story of Agdestis, who was born from the same rock as Cybele (maybe he was her son? It doesn't say.) Anyway, this rock was the same one from which Deucalion and Pyrrha took the stones that became human beings.

Agdestis was violent and uncontrollable. One day Dionysius drugged him with wine and made vines grow from his privates. Agdestis tripped and partially ripped them. From the blood, a pomengranate tree grew. The fruit made a maiden pregnant.

The child born of the maiden was named Attis, and became Cybele's favorite. Agdestis protected Attis. One day, the king of Phrygia's capital arranged for Attis to marry his daughter. Agdestis wasn't too happy about that, and ended up driving the wedding mad as well as castrating himself. Attis did the same and became very upset. That may have been the origin of the castration rites...maybe.

-That was a long post...anyway Ankh, your post reminded me of this little tidbit.