MasterCard and Diners Club have recently merged. Diners Club cards will be issued as MasterCard cards.
Commerce Server 2002 and 2000 ship a credit card number validation pipeline component that needs to be updated to reflect the new numbers issued to Diners Club. If you are running Commerce Server 2000 or Commerce Server 2002 and are validating Diner’s Club cards on your site, please contact myself or your local MS support contact for the updated pipeline component. This will be available soon as a KB article.