My friend Ewout just answered this question a couple months ago on stackoverflow. Add the following to your theme’s functions.php file:
add_filter ('add_to_cart_redirect', 'redirect_to_checkout');
function redirect_to_checkout() {
global $woocommerce;
$checkout_url = $woocommerce->cart->get_checkout_url();
return $checkout_url;
}
Here’s a link to his answer: https://stackoverflow.com/a/15597933/1543310