If you inspect the text which is outputed in the form it is actually "Credit card:", I think if you add ":" to the end of your string it should then work.
So original = Credit card:
Replacement = whatever text you want to appear instead.
You could also use a hook_form_alter in your own module instead.
<?php
function your_module_form_alter() {
if($form_id == 'uc_cart_checkout_form' && isset($form['panes']['payment']['payment_method']['#options']['credit'])){
$form['panes']['payment']['payment_method']['#options']['credit'] = 'Credit/Debit Card whatever text you want to appear';
}
}
?>