I have been using this plugin for ordering my custom taxonomy terms:
Basically, it adds a column to the 'wp_terms' table called 'term_order'. The plugin provides an interface for ordering the terms, but I don't think that is necessary.
Would want to see something like 'page-attributes' for taxonomy terms.
Note: Once you add the column 'term_order' you can instantly pass
'orderby' => 'term_order' into your favorite wp functions: query_posts, get_terms