Magento – Difference in saving of billing and shipping addresses

Let's say that you have the order id 10000015: So

$orderId = '10000015';
$order = Mage::getModel('sales/order')->loadByIncrementId($orderId);
$billingAddress = $order->getBillingAddress();
$shippingAddress = $order->getShippingAddress();

Looking at Using Javascript to validate response in the text box pointed me in the right direction.

I still have to clean up the code and integrate it correctly in firecheckout, but the following code basically does exactly what I want:

/** 
 * Check if customer email address already exists 
 */ 

public function checkEmailexistsAction()
{       

    if ($this->_expireAjax()) {
        echo 'Ajax Expired'; 
        return;
    }
    if (!$this->getRequest()->isPost()) {
        echo '!isPost'; 
        return;
    }

    $websiteId = Mage::app()->getWebsite()->getId();
    $data = $this->getRequest()->getPost('billing', array());
    $email = $data['email'];

    $customer = Mage::getModel('customer/customer');

    if ($websiteId) {
        $customer->setWebsiteId($websiteId);
        }

    $customer->loadByEmail($email);

    if ($customer->getId()) {
        // This is the part of the code that runs when a customer exists ... 
        echo 'Yup - this one exists!, email address = ' . $email . ' and ID = ' . $customer->getId(); 
        }
    else {
        // This is the part of the code that runs when a customer does not yet exist ...
        echo 'Nope.. This one is not there...'; 
    }
}

Posting 'billing[email]=someemailaddress@somehost.org' to /firecheckout/index/checkEmailexists now reliably returns a yes or no.