The illustrated example below is a mini module that automatically adds the user signature to any new forum topic that they post. However, the same technique can easily be adapted to add some default text to any new node of a specific type or to all new nodes. See the API documentation for hook_form_alter, forum_form, and comment_form to understand how this works.

As a use case, this code may satisfy those who think it's inconsistent that comments get signatures, but not the forum topic that serves as the first post in the thread.

Save the code below as forumsig.module (but remove the closing ?>).

// Tested with Drupal 4.7.2- do not use with Drupal 4.6
/**
* Implementation of hook_help().
*/
function forumsig_help($section) {
  switch ($section) {
    case 'admin/modules#description':
      return t('Adds user signature to new forum topics');
  }
}

/**
* Implementation of hook_form_alter().
*
* Adds the current user's signature as the default text for any new forum topic.
*/
function forumsig_form_alter($form_id, &$form) {
  if (isset($form['type']) && ('forum_node_form' == $form_id)) {
    global $user;   
    if ($form['body_filter']['body']['#default_value'] == ''){
      $form['body_filter']['body']['#default_value'] = $user->signature;
    }   
  }
}