How to populate a node reference field that has multiple values?
I managed to get multiple value node reference field using the Array Tokens module, an extension to the Token module, providing tokens for multiple-value CCK fields and taxonomy terms.

$authors = array([node:field_op_author-default-array-nid]);
$options = array();

foreach ($authors as $author) {
  $options[] = array('nid'=&gt;$author);
}

return($options);