Return to Revisions

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.

ADDITION:

This drupalfoo works because, instead of calling theme_lt_unified_login_page() directly, it does it like this:

$output = theme('lt_unified_login_page', $variables);

and it is the theme() function that does the magic in that when you build the theme registry, it will discover that there is a YOURTHEME_lt_unified_login_page() function out there, and will call that as an override to the theme_lt_unified_login_page() one that logintoboggan provides. (Note that the first argument doesn't start with theme_ or YOURTHEME_, these are prepended by the theme system.)