Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 12365)
+++ wp-includes/pluggable.php	(working copy)
@@ -716,7 +716,7 @@
  *
  * @since 2.0.0
  *
- * @return bool True if user is logged in, false if not logged in.
+ * @return int the user's ID if user is logged in, false if not logged in.
  */
 function is_user_logged_in() {
 	$user = wp_get_current_user();
@@ -724,7 +724,7 @@
 	if ( $user->id == 0 )
 		return false;
 
-	return true;
+	return $user->id;
 }
 endif;