It should be worldwide. Per the payment processor, "works with any US-issued and most non-US issued cards bearing a Visa, MasterCard, American Express, Discover, or JCB logo."

EDIT: I say 'should' because I have not had any overseas customers yet. Also the processor says if your international card does not process, contact the issuer because there may be an issue with currency conversion since payment can only be received in a USD amount.