Embed a node form with default values in drupal 7
If you are using a 'select list' widget for the node reference you can do this:

$form = node_add('product_plan');
$form['FIELD_NAME'][$form['language']['#value']]['#value'] = 'NID';
return drupal_render($form);


'autocomplete' widget would look like:

$form = node_add('product_plan');
$element['#default_value'] = 'NID';
$form['FIELD_NAME'][$form['language']['#value']][0]['nid']['#value'] = node_reference_autocomplete_value($element, FALSE, '');
return drupal_render($form);