The quickest way might be/would be to copy...


    /**
     * Theme function for unified login page.
     *
     * @ingroup themable
     */
    function theme_lt_unified_login_page($variables) {
    
      $login_form = $variables['login_form'];
      $register_form = $variables['register_form'];
      $active_form = $variables['active_form'];
      $output = '';
    
      $output .= '<div class="toboggan-unified ' . $active_form . '">';
    
      // Create the initial message and links that people can click on.
      $output .= '<div id="login-message">' . t('You are not logged in.') . '</div>';
      $output .= '<div id="login-links">';
      $output .= l(t('I have an account'), 'user/login', array('attributes' => array('class' => array('login-link'), 'id' => 'login-link')));
      $output .= ' ';
      $output .= l(t('I want to create an account'), 'user/register', array('attributes' => array('class' => array('login-link'), 'id' => 'register-link')));
    
      $output .= '</div>';
    
      // Add the login and registration forms in.
      $output .= '<div id="login-form">' . $login_form . '</div>';
      $output .= '<div id="register-form">' . $register_form . '</div>';
    
      $output .= '</div>';
    
      return $output;
    }

to your theme's template.php file and rename it...


    /**
     * Theme function for unified login page.
     *
     * @ingroup themable
     */
    function YOURTHEME_lt_unified_login_page($variables) {
    
      $login_form = $variables['login_form'];
      $register_form = $variables['register_form'];
      $active_form = $variables['active_form'];
      $output = '';
    
      $output .= '<div class="toboggan-unified ' . $active_form . '">';
    
      // Create the initial message and links that people can click on.
      $output .= '<div id="login-message">' . t('PLACE YOUR CUSTOM MESSAGE HERE.') . '</div>';
      $output .= '<div id="login-links">';
      $output .= l(t('I have an account'), 'user/login', array('attributes' => array('class' => array('login-link'), 'id' => 'login-link')));
      $output .= ' ';
      $output .= l(t('I want to create an account'), 'user/register', array('attributes' => array('class' => array('login-link'), 'id' => 'register-link')));
    
      $output .= '</div>';
    
      // Add the login and registration forms in.
      $output .= '<div id="login-form">' . $login_form . '</div>';
      $output .= '<div id="register-form">' . $register_form . '</div>';
    
      $output .= '</div>';
    
      return $output;
    }

...along with whatever you wanted that `login-message` to be.  Clear caches/theme registry and you should be on your way.