A walker class to use that extends wp_dropdown_categories and allows it to use the term's slug as a value rather than ID

sh-wp-dropdown-taxonomies.php

<?php

/* 
 * A walker class to use that extends wp_dropdown_categories and allows it to use the term's slug as a value rather than ID.
 *
 * See http://core.trac.wordpress.org/ticket/13258
 *
 * Usage, as normal:
 * wp_dropdown_categories($args);
 *
 * But specify the custom walker class, and (optionally) a 'id' or 'slug' for the 'value' parameter:
 * $args=array('walker'=> new SH_Walker_TaxonomyDropdown(), 'value'=>'slug', .... );
 * wp_dropdown_categories($args);
 * 
 * If the 'value' parameter is not set it will use term ID for categories, and the term's slug for other taxonomies in the value attribute of the term's <option>.
*/

class SH_Walker_TaxonomyDropdown extends Walker_CategoryDropdown{

    function start_el(&$output, $category, $depth, $args) {
        $pad = str_repeat('&nbsp;', $depth * 3);
        $cat_name = apply_filters('list_cats', $category->name, $category);

        if( !isset($args['value']) ){
            $args['value'] = ( $category->taxonomy != 'category' ? 'slug' : 'id' );
        }

        $value = ($args['value']=='slug' ? $category->slug : $category->term_id );

        $output .= "\t<option class=\"level-$depth\" value=\"".$value."\"";
        if ( $value === (string) $args['selected'] ){ 
            $output .= ' selected="selected"';
        }
        $output .= '>';
        $output .= $pad.$cat_name;
        if ( $args['show_count'] )
            $output .= '&nbsp;&nbsp;('. $category->count .')';

        $output .= "</option>\n";
        }

}

Wonderful, saved my time. Than you for sharing the code.