<a href="<?php echo get_permalink(get_page_by_path( 'checkout' )); ?>"><span id="theme-checkout-count"><?php echo $cart_count = wpsc_cart_item_count(); ?></span></a>
function theme_cart_update() {
$cart_count = wpsc_cart_item_count();
echo <<<HTML
jQuery("#theme-checkout-count").html("$cart_count");
HTML;
}
add_action('wpsc_alternate_cart_html', 'theme_cart_update');
https://snippets.webaware.com.au/snippets/add-custom-javascript-actions-to-wp-e-commerce-cart-updates/
