Using hook_form_alter to set default values for multiple-value CCK fields
I think you may need to use #after_build, to ensure that your alter code runs after CCK has completely built the form element. Try something like this

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['field_myfield']['#after_build'][] = 'mymodule_after_build';
}

function mymodule_after_build(&amp;$form_element, &amp;$form_state) {
  // Have a look at the element with devel's dsm
  dsm($form_element);

  // Code to adjust your form element here...

  return $form_element;
}


I'm not 100% on the above syntax. I'd sugest you have a look at this article entitled hook_form_alter() and CCK fields