Testing for taxonomy
There is a Drupal Dojo video that answers this exact question. It is done in Drupal 6 but might give you some clues how to accomplish this in Drupal 7.

(edit: Response to comment) Although it might add complexity to developing your website, you can use the Context module to trigger block visibility based on certain criteria such as a View being viewed or a taxonomy term page being viewed. Keep in mind that if the Context module is used with all blocks disabled in the block UI it gives a performance boost to the website.