/**
 * Implementation of hook_comment().
 *
 * Append signatures to comments, if the parent node type is signature-enabled.
 */
function user_comment($comment, $op) {
  if ($op == 'view') {
    $node = node_load($comment->nid);
    if (variable_get('user_signatures_'. $node->type, 0)) {
      $comment->comment .= theme('user_signature', $comment->user_signature);
    }
  }
}