Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 14337)
+++ wp-includes/theme.php	(working copy)
@@ -7,6 +7,17 @@
  */
 
 /**
+ * Whether a child theme is in use.
+ *
+ * @since 3.0.0
+ *
+ * @return bool true if a child theme is in use, false otherwise.
+ **/
+function is_child_theme() {
+	return ( TEMPLATEPATH !== STYLESHEETPATH );
+}
+
+/**
  * Retrieve name of the current stylesheet.
  *
  * The theme name that the administrator has currently set the front end theme