Index: wp-includes/canonical.php
===================================================================
--- wp-includes/canonical.php	(revision 19740)
+++ wp-includes/canonical.php	(working copy)
@@ -426,8 +426,14 @@
  *		location on success.
  */
 function redirect_guess_404_permalink() {
-	global $wpdb;
+	global $wpdb, $wp_rewrite;
 
+	if ( preg_match( '#^[^%]+%category%#', $wp_rewrite->permalink_structure ) 
+		&& $cat = get_category_by_path( $_SERVER['REQUEST_URI'] )
+	) {
+		return get_term_link( $cat );
+	}
+
 	if ( !get_query_var('name') )
 		return false;