hook_form_alter selecting a default value in a node reference select list
CCK requires a little voodoo to alter values with a hook_form_alter(). You need to implement a special type: after_build, so put this code in a custom.module:

//Declare the callback for after_build for page node form
function custom_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'page_node_form) {
    $form['#after_build'][] = 'custom_after_build';
  }
}

function custom_after_build($form, &amp;$form_state) {
  //Set my node ref to 5
  $form['field_node']['nid']['nid']['#value'][0] = 5;
  //you can set more than one using 
  $form['field_node']['nid']['nid']['#value'][2] = 6;
  //have a look at the form after after_build
  dsm($form);
  return $form;
}


I adapted a previous answer of mine, but I have to admin that node reference is even more confusing ;)