Today my company, iHwy, released an enhanced credit card validation extension for the jQuery Validation plugin. The extension provides a way for a developer to pass in the type of credit card being used (Amex, Visa, Mastercard and a bunch of others) so that the validation extension can check the numbers prefixing the credit card number and verify that the length of the card number is correct. It also does the mod-10 validation that the core creditcard validation routine does.

http://www.ihwy.com/labs/jquery-validate-credit-card-extension.aspx

That page shows some examples of card numbers that incorrectly pass the core creditcard validation routine, but do not pass when the extension is used. This extension should make it pretty easy for you to tighten up your credit card validation. The sample code we've provided shows a way to bind your credit card type dropdown selection to the validation extension.