Custom Taxonomies and current taxonomy term ID

I hope someone may be able to help me here.

I have followed the excellent tutorial series here:
http://justintadlock.com/archives/2009/05/06/custom-taxonomies-in-wordpress-28
and here:
http://justintadlock.com/archives/2009/06/04/using-custom-taxonomies-to-create-a-movie-database

I find myself needing to retrieve the current custom taxonomy term ID similar how it is done here for tags:
http://wordpress.org/support/topic/305892?replies=4

what I’m using for tags is this inside the tag.php

$term_id 			= get_query_var('tag_id');
$taxonomy 			= 'post_tag';
$args 				='include=' . $term_id;
$terms 				= get_terms( $taxonomy, $args );

Now I need to do the same for my custom taxonomy but I can’t seem to nail it. What I don’t understand is the tag_id in get_query_var(‘tag_id’) is coming from where? Where can I find a list of the variable keys the get_query_var() function can take?