We are using the Authorize.net AIM interface in Magento in a slightly different way and need to remove the fields for card type, credit card number, expire date and CVV field. We are using the Authorize.net code to send to a page that will approve and disapprove transactions based on name, so the CC info isn't needed.
The fields need to be removed or made invisible with defaults so that Authorize.net processing will proceed, and not get stopped by the verifier.
We have traced through the code and found the getPaymentMethodFormHtml(Mage_Payment_Model_Method_Abstract $method) that seems to load the form information using getChildHtml('payment.method.' . $method->getCode()), but we have been unable to trace it down from there.