Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 18407)
+++ wp-includes/query.php	(working copy)
@@ -2315,6 +2315,8 @@
 			$orderby = "$wpdb->posts.post_date " . $q['order'];
 		} elseif ( 'none' == $q['orderby'] ) {
 			$orderby = '';
+		} elseif ( $q['orderby'] == 'post__in' && ! empty( $post__in ) ) {
+			$orderby = "find_in_set(ID, '" . $post__in . "')"; 
 		} else {
 			// Used to filter values
 			$allowed_keys = array('author', 'date', 'title', 'modified', 'menu_order', 'parent', 'ID', 'rand', 'comment_count');