Index: wp-includes/category.php
===================================================================
--- wp-includes/category.php	(revision 9036)
+++ wp-includes/category.php	(working copy)
@@ -40,7 +40,7 @@
 	$defaults = array( 'type' => 'category' );
 	$args = wp_parse_args( $args, $defaults );
 
-	$taxonomy = 'category';
+	$taxonomy = apply_filters('get_categories_taxonomy','category',$args);
 	if ( 'link' == $args['type'] )
 		$taxonomy = 'link_category';
 	$categories = (array) get_terms( $taxonomy, $args );