Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 23400)
+++ wp-includes/formatting.php	(working copy)
@@ -908,7 +908,7 @@
 
 	// If strict, reduce to ASCII for max portability.
 	if ( $strict )
-		$username = preg_replace( '|[^a-z0-9 _.\-@]|i', '', $username );
+		$username = preg_replace( '|[^a-z0-9+ _.\-@]|i', '', $username );
 
 	$username = trim( $username );
 	// Consolidate contiguous whitespace