Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 23297)
+++ wp-includes/ms-functions.php	(working copy)
@@ -1326,10 +1326,13 @@
 /**
  * Get a numeric user ID from either an email address or a login.
  *
+ * A numeric string is considered to be an existing user ID
+ * and is simply returned as such.
+ *
  * @since MU
  * @uses is_email()
  *
- * @param string $string
+ * @param string $string Either an email address or a login.
  * @return int
  */
 function get_user_id_from_string( $string ) {