Basically but invoking an instance of the front controller its possible to grab the router and assemble a url. Assemble is the function used in the view helper. The URL is built up from an array of module, controller, action, etc, followed by a second parameter of the route to use. The code is as follows:

<?php
/**
 * View helper which returns link category URL
 *
 * @author     Lloyd Watkin
 * @since      25/01/2010
 * @package    ViewHelper
 * @subpackage LinksUrl
 */
class Pro_View_Helper_LinksUrl
    extends Zend_View_Helper_Abstract
{
	/**
	 * Returns link category URL
	 *
	 * @param  Doctrine_Record $category
	 * @param  string          $module
	 * @param  string          $controller
	 * @param  string          $action
	 * @return string Url
	 */
    public function linksUrl($category, $module = 'www',
        $controller = 'links', $action = 'index')
    {
    	$router = Zend_Controller_Front::getInstance()->getRouter();

        return $router->assemble(array(
            'module'     => $module,
            'controller' => $controller,
            'action'     => $action,
            'category'   => "{$category->id}-{$category->name}",
        ), 'www-index');
    }
}

Another way to do this is to invoke Zend_View_Helper_Url itself and call the Url method (if you want to use the helper itself). This can be done by using the following code:

<?php
/**
 * View helper which returns link category URL
 *
 * @author     Lloyd Watkin
 * @since      25/01/2010
 * @package    ViewHelper
 * @subpackage LinksUrl
 */
class Pro_View_Helper_LinksUrl
    extends Zend_View_Helper_Abstract
{
	/**
	 * Returns link category URL
	 *
	 * @param  Doctrine_Record $category
	 * @param  string          $module
	 * @param  string          $controller
	 * @param  string          $action
	 * @return string Url
	 */
    public function linksUrl($category, $module = 'www',
        $controller = 'links', $action = 'index')
    {
    	$link = new Zend_View_Helper_Url();

        return $link->url(array(
            'module'     => $module,
            'controller' => $controller,
            'action'     => $action,
            'category'   => "{$category->id}-{$slug}",
        ), 'www-index');
    }
}

Both almost identical. Not a hard thing to do in the framework but can catch you out ;)