How to alter commerce checkout form in a custom module?
I think you're looking for commerce_checkout_form() (commerce_checkout_form_checkout doesn't exist as far as I can tell from greping Commerce's code):

function MYMODULE_form_commerce_checkout_form_alter($form, &amp;$form_state, $form_id) {
  ...
}


or

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  switch ($form_id) {
    case 'commerce_checkout_form':
      ...
    break;
}