Create a body class for a specific node type based on the value of a field on the node.

template.php

<?
function themename_preprocess_html(&$variables) {
  // Add a body class to Articles based on the value of the 'field_article_type'
  // field. e.g.: 'article-type-news' or 'article-type-blog'.
  if(!empty($variables['page']['content']) && !empty($variables['page']['content']['system_main']['content']['nodes'])) {
    $node = $variables['page']['content']['system_main']['content']['nodes'][arg(1)]['body']['#object'];
    if(user_is_logged_in()) {
      $node = $variables['page']['content']['system_main']['content']['nodes'][arg(1)]['#node'];
    }
    if($node->type == 'article' && !empty($node->field_article_type)) {
      $type = field_get_items('node', $node, 'field_article_type');
      $type_value = field_view_value('node', $node, 'field_article_type', $type[0]);
      $variables['classes_array'][] = 'article-type-' . strtolower(render($type_value));
    }
  }
}

?>