Looks like all of the credit processing in the iTunes store, credit cards included, are down.
Twitter Search currently shows tons of people having this issue and it seems to have started about 20 hours ago. The error is a pop-up message that shows up anytime you try to buy something…
Due to a problem with the iTunes Store, your store credit balance is temporarily unavailable for use in purchasing music; you may continue purchasing using a different method of payment.
As it is the weekend, no one is available over at iTunes for comment. Hopefully, they’ll have this resolved shortly.