add_filter( 'add_to_cart_text', 'woo_custom_cart_button_text' );                                // < 2.1
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woo_custom_cart_button_text' );    // 2.1 +

function woo_custom_cart_button_text() {

        return __( 'My Button Text', 'woocommerce' );

}