Index: wp-includes/category-template.php
===================================================================
--- wp-includes/category-template.php	(revision 10234)
+++ wp-includes/category-template.php	(working copy)
@@ -717,8 +717,13 @@
  * @see Walker_Category::walk() for parameters and return description.
  */
 function walk_category_tree() {
-	$walker = new Walker_Category;
 	$args = func_get_args();
+	// the user's options are the third parameter
+	if ( empty($args[2]['walker']) || !is_a($args[2]['walker'], 'Walker') )
+		$walker = new Walker_Category;
+	else
+		$walker = $args[2]['walker'];
+
 	return call_user_func_array(array( &$walker, 'walk' ), $args );
 }
 
@@ -730,8 +735,13 @@
  * @see Walker_CategoryDropdown::walk() for parameters and return description.
  */
 function walk_category_dropdown_tree() {
-	$walker = new Walker_CategoryDropdown;
 	$args = func_get_args();
+	// the user's options are the third parameter
+	if ( empty($args[2]['walker']) || !is_a($args[2]['walker'], 'Walker') )
+		$walker = new Walker_CategoryDropdown;
+	else
+		$walker = $args[2]['walker'];
+
 	return call_user_func_array(array( &$walker, 'walk' ), $args );
 }
 
Index: wp-admin/includes/template.php
===================================================================
--- wp-admin/includes/template.php	(revision 10234)
+++ wp-admin/includes/template.php	(working copy)
@@ -439,8 +439,10 @@
  * @param unknown_type $selected_cats
  * @param unknown_type $popular_cats
  */
-function wp_category_checklist( $post_id = 0, $descendants_and_self = 0, $selected_cats = false, $popular_cats = false ) {
-	$walker = new Walker_Category_Checklist;
+function wp_category_checklist( $post_id = 0, $descendants_and_self = 0, $selected_cats = false, $popular_cats = false, $walker = null ) {
+	if ( empty($walker) || !is_a($walker, 'Walker') )
+		$walker = new Walker_Category_Checklist;
+
 	$descendants_and_self = (int) $descendants_and_self;
 
 	$args = array();