Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 16200)
+++ wp-includes/post-template.php	(working copy)
@@ -357,6 +357,13 @@
 		$classes[] = 'tag-' . sanitize_html_class($tag->slug, $tag->term_id);
 	}
 
+	// Post Formats
+	$post_format = get_post_format( $post );
+	if ( $post_format && !is_wp_error($post_format) )
+		$classes[] = 'post-format-' . esc_attr( $post_format );
+	else
+		$classes[] = 'post-format-default';
+
 	if ( !empty($class) ) {
 		if ( !is_array( $class ) )
 			$class = preg_split('#\s+#', $class);