Index: wp-includes/classes.php
===================================================================
--- wp-includes/classes.php	(revision 5571)
+++ wp-includes/classes.php	(working copy)
@@ -491,9 +491,12 @@
 	var $tree_type = 'page';
 	var $db_fields = array ('parent' => 'post_parent', 'id' => 'ID'); //TODO: decouple this
 
-	function start_lvl($output, $depth) {
+	function start_lvl($output, $depth, $current_page) {
 		$indent = str_repeat("\t", $depth);
-		$output .= "\n$indent<ul>\n";
+		$_current_page = get_page( $current_page );
+		$output .= "\n$indent<ul";
+		if ( $depth == 1 && $_current_page->post_parent != 0 ) $output .= ' class="current_page_tree"';
+		$output .= ">\n";
 		return $output;
 	}