First you'll have to make use of hook_form_alter() function.
function example_form_alter(&$form, $form_state, $form_id) {
$form['field_example_reference']['#pre_render'] = array('example_change_node_reference');
}
Now the node reference will look for a example_change_node_reference() function which will return an array of options.
function example_change_node_reference($element) {
$element['nid']['nid']['#options'] = array($node->;nid => $node->title);
return $element;
}
Make sure you pass the $element variable into the function. More info:http://drupal.org/node/339730.
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