Add a class to widgets in a specific sidebar

gistfile1.aw

<?php

/**
 * Add 'class="span4"' to all widgets in the Content Bottom sidebar
 */
function bb_content_bottom_widget_class($params) {
  if ($params[0]['id'] == 'roots-content-bottom') {
    $class = 'class="span4 ';
    $params[0]['before_widget'] = preg_replace('/class=\"/', "$class", $params[0]['before_widget'], 1);
  }

  return $params;
}

add_filter('dynamic_sidebar_params', 'bb_content_bottom_widget_class');

?>