Hi Kate, me again
Just comparing a number of plugins that query taxonomies.
With yours, it looks like you need to change line 109 to use the term slug, rather than the term name. (If slug and name the same it will not matter, but when they are different, of course it does.... )
$option_name = $taxonomy->name.'='.$term->slug;
as comparison to your plugin, see http://wordpress.org/extend/plugins/query-multiple-taxonomies/
I think yours would also be more helpful if it remembered (maybe from the query string) the values chosen already and then allowed the user to progressively drill down.