Index: /Users/denis/Sites/sem-pro/wp-includes/registration.php
===================================================================
--- /Users/denis/Sites/sem-pro/wp-includes/registration.php	(revision 1218)
+++ /Users/denis/Sites/sem-pro/wp-includes/registration.php	(working copy)
@@ -176,6 +176,18 @@
 	if ( empty($user_registered) )
 		$user_registered = gmdate('Y-m-d H:i:s');
 
+	$user_nicename_check = $wpdb->get_var( $wpdb->prepare("SELECT ID FROM $wpdb->users WHERE user_nicename = %s AND ID != %d LIMIT 1" , $user_nicename, (int) $ID));
+
+	if ($user_nicename_check) {
+		$suffix = 2;
+		while ($user_nicename_check) {
+			$alt_user_nicename = $user_nicename . "-$suffix";
+			$user_nicename_check = $wpdb->get_var( $wpdb->prepare("SELECT ID FROM $wpdb->users WHERE user_nicename = %s AND ID != %d LIMIT 1" , $alt_user_nicename, (int) $ID));
+			$suffix++;
+		}
+		$user_nicename = $alt_user_nicename;
+	}
+	
 	$data = compact( 'user_pass', 'user_email', 'user_url', 'user_nicename', 'display_name', 'user_registered' );
 	$data = stripslashes_deep( $data );