Class to extended WordPress' 3.0.x WP_Query to allow querying for all terms in a taxonomy.

posts-by-taxonomy.php

<?php
/*
    PostsByTaxonomy class that extends WP_Query and filters to includes all posts that have any term of a taxonomy.

    Author: Mike Schinkel (http://mikeschinkel.com)

    This example works, just drop into the root of your website and call directly.
    Use the class in your plugins or themes.

    See: 
      http://wordpress.stackexchange.com/questions/3708/custom-taxonomy-wp-query-for-all-terms-in-a-taxonomy
      http://lists.automattic.com/pipermail/wp-hackers/2010-November/035730.html

*/

header('Content-type:text/plain');
include "wp-load.php";

class PostsByTaxonomy extends WP_Query {
  var $posts_by_taxonomy;
  var $taxonomy;
  function __construct($args=array()) {
    add_filter('posts_join',array(&$this,'posts_join'),10,2);
    $this->posts_by_taxonomy = true;
    $this->taxonomy = $args['taxonomy'];
    unset($args['taxonomy']);
    parent::query($args);
  }
  function posts_join($join,$query) {
    if (isset($query->posts_by_taxonomy)) {
      global $wpdb;
      $join .=<<<SQL
INNER JOIN {$wpdb->term_relationships} ON {$wpdb->term_relationships}.object_id={$wpdb->posts}.ID
INNER JOIN {$wpdb->term_taxonomy} ON {$wpdb->term_taxonomy}.term_taxonomy_id={$wpdb->term_relationships}.term_taxonomy_id
  AND {$wpdb->term_taxonomy}.taxonomy='{$this->taxonomy}'
SQL;
    }
    return $join;
  }
}

$query = new PostsByTaxonomy(array(
  'taxonomy' => 'category',
));
foreach($query->posts as $post) {
  echo " {$post->post_title}\n";
}