don’t include WP like that. use $_GET instead:
...
$("#tags").autocomplete("<?php echo add_query_arg('get_my', 'terms', home_url()); ?>",
...
theme’s functions.php:
add_action('template_redirect', 'terms_for_autocomplete');
function terms_for_autocomplete(){
if(isset($_GET['get_my']) && $_GET['get_my'] == 'terms'):
$terms = &get_terms(get_taxonomies());
foreach ($terms as $term)
echo "{$term->name}|{$term->name} ({$term->count} results)\n";
die();
endif;
}