Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 17488)
+++ wp-includes/taxonomy.php	(working copy)
@@ -22,7 +22,7 @@
 		'rewrite' => did_action( 'init' ) ? array(
 					'hierarchical' => true,
 					'slug' => get_option('category_base') ? get_option('category_base') : 'category',
-					'with_front' => false) : false,
+					'with_front' => ( get_option('category_base') ) ? false : true ) : false,
 		'public' => true,
 		'show_ui' => true,
 		'_builtin' => true,
@@ -34,7 +34,7 @@
 		'query_var' => 'tag',
 		'rewrite' => did_action( 'init' ) ? array(
 					'slug' => get_option('tag_base') ? get_option('tag_base') : 'tag',
-					'with_front' => false) : false,
+					'with_front' => ( get_option( 'tag_base' ) ) ? false : true ) : false,
 		'public' => true,
 		'show_ui' => true,
 		'_builtin' => true,