I'd watch out for copyright infringements. Not sure if you can actual register an acronym, but it wouldn't put it past Sun to do that. So printing up shirts with SCJP would be illegal, if that is the case.
I don't believe there is any protection for the letters scjp which could as easily stand for Simon Cat Just Pee'd, but if you were to use the word Java or say Sun Certified Java Programmer or similar then you might have an issue. Of course if Sun were a little more customer oriented they would include an offer of such merchandise with the mail that sent your certificate.