Checkout Extensions – Adding Customer Comment Box During Checkout

You can try this logic.
Get the first order of the customer.
Get the quote object of the order.
Check the checkout method of the quote.

Something like this:

$customerId = Mage::getSingleton('customer/session')->getCustomer()->getId()
$orders = Mage::getModel('sales/order')->getCollection()
    ->addFieldToFilter('customer_id', $customerId)
    ->setOrder('entity_id', 'ASC')
    ->setCurPage(1)
    ->setPageSize(1); //limit to 1;
$firstOrder = $orders->getFirstItem(); 

if (!$firstOrder->getId()) {
   //it means the customer did not place any orders so he did not register at checkout
}
else {
    $quoteId = $firstOrder->getQuoteId();
    $quote = Mage::getModel('sales/quote')->load($quoteId);
    $checkoutMethod = $quote->getCheckoutMethod();
    if ($checkoutMethod == 'register') {
        //customer registered at checkout
    }
    else {
       //customer did not register at checkout
    }
}

I didn't test the code but the idea seams ok to me.

You'll want to create an event-observer module & observe sales_order_place_after.

Your observer method would look like this:

public function logShipping($observer){
    $order = $observer->getEvent()->getOrder();
    $history = $order->addStatusHistoryComment($order->getShippingMethod(), false);
    $history->setIsCustomerNotified(false);
    $order->save();
}