function fb_add_body_class( $class ) {

	if ( ! is_tax() )
		return $class;

	$tax   = get_query_var( 'taxonomy' );
	$term  = $tax . '-' . get_query_var( 'term' );
	$class = array_merge( $classes, array( 'taxonomy-archive', $tax, $term ) );

	return $class;
}
add_filter( 'body_class', 'fb_add_body_class' ); // or post_class as hook