create multiple nodes from title field of one node add form

This may not work, but it's an idea you could try...

My approach would be this: the user is creating a node with "a,b,c,d" in the title and a node reference field that points to X. We want 4 nodes with titles a, b, c, and d all with node references that point to X. Let's try using hook_nodeapi to perform the creation of these extra nodes when this original node is getting saved. Also, lets let that original node get saved, but it will only be creating the first item in the series - a in this case. We'll have some PHP code create nodes b,c,d as copies.

function mymodulename_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {

  // mynodetype below is the system name of the content type you want this to work with

  if (($node->type == 'mynodetype') && ($op == 'prepare')) {

    $titles_array = explode(',', $node->title);

    $title_index = 1;
    foreach($titles_array as $current_title) {

      if ($title_index == 1) {
        $node->title = $current_title;  // this node will represent the first term in the title, so set it here
      } else {  // we need to make a copy

        $clone = $node;  // need to test this, i am not clear on references with assignment
        $clone->is_new = true;
        $clone->title = $current_title;
        unset($clone->nid);
        unset($clone->vid);
        drupal_save($clone);

      }      
      $title_index++;
    }

  }

}

Definitely test and clean it up, but hopefully this gets you started.