Index: wp-admin/edit-form-ajax-cat.php
===================================================================
--- wp-admin/edit-form-ajax-cat.php	(revision 2779)
+++ wp-admin/edit-form-ajax-cat.php	(working copy)
@@ -12,8 +12,11 @@
 	$new_cat_id = func_get_arg(0);
 }
 
+function get_out_now() { exit; }
+
 add_action('edit_category', 'grab_id');
 add_action('create_category', 'grab_id');
+add_action('shutdown', 'get_out_now', -1);
 
 $cat_name = stripslashes($_GET['ajaxnewcat']);
 
@@ -25,5 +28,5 @@
 $cat_name = $wpdb->escape($cat_name);
 $cat_array = compact('cat_name', 'category_nicename');
 wp_insert_category($cat_array);
-echo $new_cat_id;
-?>
\ No newline at end of file
+die($new_cat_id);
+?>
Index: wp-admin/admin-header.php
===================================================================
--- wp-admin/admin-header.php	(revision 2779)
+++ wp-admin/admin-header.php	(working copy)
@@ -159,7 +159,7 @@
 
 function newCatCompletion() {
 	var p = getResponseElement();
-	var id = ajaxCat.response;
+	var id = parseInt(ajaxCat.response, 10);
 	if ( id == '-1' ) {
 		p.innerHTML = "You don't have permission to do that.";
 		return;