Load a different theme for IE less than 9

Drop this inside a plugin:

add_filter('template', function($template){

  // could be IE
  if(preg_match('/MSIE [1-9]/i', $_SERVER['HTTP_USER_AGENT']))
    return 'your_ie_theme_name';

  // other
  return $template;

});

Just so you know, browser detection trough PHP is not really that accurate. There’s a get_browser() function available PHP, not sure if it’s any better though.