filter vocabulary from vocabulary
Essentially, you want a hierarchical 'drill down' through taxonomy to narrow a search? If I'm interpreting this correctly, you're looking for something like Hierarchical Select, a module that does just that in the form of an input widget. I don't have personal experience with the exposed filter implementation, but it sounds like it exists.

http://drupal.org/project/hierarchical_select

http://drupal.org/node/720268