From the rel-tag spec (http://microformats.org/wiki/rel-tag ):

"Tags are embedded in HTTP URIs in a well-defined manner so that the tag
embedded in an HTTP URI can be mechanically extracted from that URI.
Specifically, the last segment of the path portion of the URI (after the
final "/" character) contains the tag value."

In the world of Microformats there is only one url segment after the /
which properly encodes a tag/category/keyword/term/etc; adding a
textfield to control that wouldn't be too useful since there is only one
option for compliance with this spec.

I would like to see core have this without the addition of any UI,
probably using the approach laid out in http://drupal.org/node/63635 for
nodes on the code side. The URLs can be '.../{vocabulary}(/{parent
category})*/{category}'.

And since we are coding Drupal here, might as well throw in an API for a
pathauto-like module to go and change everything up. A module could Form
API in a text field for manual editing if someone wants that.

(A system to manage redirects would be nice so URLs get redirects as
they are changed.)

--
Neil Drumm
http://delocalizedham.com/

In the beginning of 2006 I have started to develop module called friendly_url. Its features are:

* Extends custom_url_rewrite functionality.

* Friendly user URLs: user/1.* --> user/root.* (e.g. user/1/edit --> user/root/edit).
root - was the name of user. If friendly_url module is enabled then users can't register with login names that will be encoded in URL.

* Nodes URLs: When user adds new node he can specify a friendly URL - which can't be number or some of the forbidden names (forbidden URLs are fetched using hook_url_rewrite_forbidden().)

* Taxonomy URLs: same as nodes.

* Taxonomy_menu URLs.

* Friendly URL caching.

Taxonomy_menu provides special rewrite from taxonomy/term/* to hierarchical taxonomy_menu/<voc_name>/<term_friendly_url>/<term_friendly_url>. Also taxonomy_menu/<voc_name>/<term_name>/<term_name>/depth/0 etc. was implemented for feeds.

Also, taxonomy_menu can be striped from the URL.

e.g. I have a taxonomy_menu/articles/artificial-intelligence/neural-networks. ---> articles/artificial-intelligence/neural-networks.

(where articles is the friendly url of vocabulary.)

Maybe this ways can be helpful for developing a hierarchical taxonomy URLs.