Another thought might be that the cards are registered outside of the US. Might that also produce this same result? Currently all Ebook purchases must be made with a card that has a mailing address inside the US.
A_Wilson, that's not necessarily true. Since Nooks are being sold overseas now, it will take some foreign credit cards. But, the overseas units make you choose your location and then would only allow an overseas CC (or so I hear), while the US nooks require a US CC.