Oh sorry, I've made a mistake
I've recently contacted Prometric
for scheduling an IBM exam, the
Prometric ID was enough.
For Sun it's indeed different. You
have to call your local Sun
representative office and ask for
a voucher, this will be sent by mail,
after the payment is received
(snail-mail). After you have received
the voucher you call Prometric or
indeed book online.
Sorry for the confusion.
Gian