Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 26908)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -485,6 +485,10 @@
 	if ( strlen( $user_name ) < 4 )
 		$errors->add('user_name',  __( 'Username must be at least 4 characters.' ) );
 
+	if ( strlen( $user_name ) > 60 ) {
+		$errors->add( 'user_name', __( 'Username cannot be longer than 60 characters.' ) );
+	}
+
 	if ( strpos( ' ' . $user_name, '_' ) != false )
 		$errors->add( 'user_name', __( 'Sorry, usernames may not contain the character &#8220;_&#8221;!' ) );