Hmm, I don't recall to have a stamp even on my SJCP certificate. So consider yourself lucky!
I had another interesting observation: my SCJP card (the credit card-like thing, which I thought would come in handly to impress members of the opposite sex, but it doesn't quite live up to my expectations
) bears the signature of Sun Certification Services' director, whilst my SCJD card has the signature of the Sun Educational Services' Vice President. Since it looks like they just print these things, it makes me wonder why they bothered to print different signatures.
Frans.