This is just an update so that people don't have to wade through the entire thread.
To make both category and tag slugs editable (in WPMU 2.6.1):
wp-admin/edit-category-form.php
To make the input box for "Category Slug" in the admin interface, add the following between the table rows for "Category Name" and "Category Parent":
<tr class="form-field">
<th scope="row" valign="top"><label for="category_nicename"><?php _e('Category Slug') ?></label></th>
<td><input name="category_nicename" id="category_nicename" type="text" value="<?php echo attribute_escape($category->category_nicename); ?>" size="40" />
<?php _e('The “slug” is the URL-friendly version of the name. It is usually all lowercase and contains only letters, numbers, and hyphens.'); ?></td>
</tr>
wp-admin/includes/mu.php
To stop overwriting the slug to always match the name, comment out line 384:
function sync_slugs( $term, $taxonomy, $args ) {
$args[ 'slug' ] = sanitize_title( $args[ 'name' ] );
return $args;
}
//add_filter( 'pre_update_term', 'sync_slugs', 10, 3 );