Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 12134)
+++ wp-includes/taxonomy.php	(working copy)
@@ -194,6 +194,28 @@
 	$wp_taxonomies[$taxonomy] = (object) $args;
 }
 
+/**
+ * Do the opposite of register_taxonomy()
+ */
+function unregister_taxonomy( $taxonomy, $object_type = '') {
+	global $wp_taxonomies;
+
+	if ( !isset($wp_taxonomies[$taxonomy]) )
+		return;
+
+	if ( !empty( $object_type ) ) {
+		$i = array_search($object_type, $wp_taxonomies[$taxonomy]->object_type);
+
+		if ( false !== $i )
+			unset($wp_taxonomies[$taxonomy]->object_type[$i]);
+
+		if ( empty($wp_taxonomies[$taxonomy]->object_type) )
+			unset($wp_taxonomies[$taxonomy]);
+	} else {
+		unset($wp_taxonomies[$taxonomy]);
+	}
+}
+
 //
 // Term API
 //