Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 29725)
+++ wp-includes/theme.php	(working copy)
@@ -122,6 +122,8 @@
 
 /**
  * Whether a child theme is in use.
+ * Check for a difference in stylesheet and template values, plus a non-empty
+ * "Template" comment in the child theme stylesheet.
  *
  * @since 3.0.0
  *
@@ -128,7 +130,11 @@
  * @return bool true if a child theme is in use, false otherwise.
  **/
 function is_child_theme() {
-	return ( TEMPLATEPATH !== STYLESHEETPATH );
+	$theme = wp_get_theme();
+
+	$template = $theme->get( 'Template' );
+
+	return ( TEMPLATEPATH !== STYLESHEETPATH && ! empty( $template ) );
 }
 
 /**