Excerpt from wp-e-commerce/wpsc-core/js/wp-e-ecommerce.js

        var country_drop_downs = jQuery( selector );

        country_drop_downs.empty();

        country_drop_downs.append( new Option( wpsc_var_get( 'no_country_selected' ), '' ) );
        for ( var isocode in wpsc_acceptable_shipping_countries ) {
            if ( wpsc_acceptable_shipping_countries.hasOwnProperty( isocode ) ) {
                var country_name = wpsc_acceptable_shipping_countries[isocode];
                country_drop_downs.append( new Option( country_name, isocode ) );
            }
        }

        country_drop_downs.val( current_shipping_country );