Aye, you can also look into hook_form_alter() and add a validate handler for the checkout form. It's a little tricky since the CC details are going to be in $_POST and not the Drupal forms right now. There's a little bit of trouble w/ the way payment details form fields cause errors. I'm working on a solution... read more about it in here somewhere.

I wrote a custom validation...

function shopping_form_alter($form_id, &$form) {
if (strcmp('uc_cart_checkout_form', $form_id) != 0) return;
$form['#validate'] = array('credit_check_expdate' => array());
}

But at the end of my validation, I called
uc_cart_checkout_form_validate($form_id, $form_values)

Is this OK way to do this, could I have changed my form alter so BOTH validations would have been called?
JIm