Alright, I worked out a solution:
function _show_blog() {
global $cache_userdata,
$wpdb,
$wp_query,
$wp_rewrite,
$post,
$category_cache,
$request,
$posts_per_page,
$wpdb,
$max_num_pages;
define('WP_USE_THEMES', true);
require('wordpress/wp-blog-header.php');
}
Works for me with WP1.5. Now, someone please sack the WP developers for horrible abuse of global variables.