--- sites/blogs/wp-includes/ms-functions.php	2012-03-04 19:47:47.000000000 -0300
+++ /Users/gustavobordoni/tmp/ms-functions.php	2012-03-04 20:10:25.000000000 -0300
@@ -437,7 +437,7 @@
 	$orig_username = $user_name;
 	$user_name = preg_replace( '/\s+/', '', sanitize_user( $user_name, true ) );
 	$maybe = array();
-	preg_match( '/[a-z0-9]+/', $user_name, $maybe );
+	preg_match( apply_filters( 'wpmu_username_restriction', '/[a-z0-9]+/' ), $user_name, $maybe );
 
 	if ( $user_name != $orig_username || $user_name != $maybe[0] ) {
 		$errors->add( 'user_name', __( 'Only lowercase letters (a-z) and numbers are allowed.' ) );