Here's an interesting article from Time Magazine:

http://www.time.com/time/world/article/0,8599,1891230,00.html

Like the subject header says, can someone really be "de-baptized"? It seems to me, like the bishop says in the article, it's really something between someone and God, or, if you don't believe in God, in a person's own heart and mind.

Why then the need for a certificate?