Index: node.pages.inc
===================================================================
--- node.pages.inc	(revision 3897)
+++ node.pages.inc	(working copy)
@@ -262,7 +262,10 @@
 function node_body_field(&$node, $label, $word_count) {
 
   // Check if we need to restore the teaser at the beginning of the body.
-  $include = !isset($node->teaser) || ($node->teaser == substr($node->body, 0, strlen($node->teaser)));
+  // We can safely ignore whitespaces here as they are not reflected on the actual HTML display.
+  $teaser = trim(preg_replace('/\s+/', ' ', $node->teaser));
+  $body = trim(preg_replace('/\s+/', ' ', substr($node->body, 0, strlen($node->teaser))));  
+  $include = !isset($node->teaser) || ($teaser == $body);  
 
   $form = array(
     '#after_build' => array('node_teaser_js', 'node_teaser_include_verify'));