class Namespace_Module_Model_Observer {

        public function subscribedToNewsletter(Varien_Event_Observer $observer)

    {

        $event = $observer->getEvent();

        $subscriber = $event->getDataObject();

        $data = $subscriber->getData();

        $email = $data['subscriber_email'];

        $statusChange = $subscriber->getIsStatusChanged();

        if ($data['subscriber_status'] == "1" && $statusChange == true) {

                        //code to handle if customer is just subscribed...

                }

        }

}

class Namespace_Module_Model_Observer {

  public function customerRegisterSuccess(Varien_Event_Observer $observer) {

      $event = $observer->getEvent();

      $customer = $event->getCustomer();

      $email = $customer->getEmail();

      if($email) {

          //code to handle if customer is successfully registered

      }

  }

}

<?php $order = Mage::getModel('sales/order')->loadByIncrementId($this->getOrderId());

$quoteId = $order->getQuoteId();

$quote = Mage::getModel('sales/quote')->load($quoteId);

$method = $quote->getCheckoutMethod(true);

$customer_email = $order->getCustomerEmail();

if ($method == 'register'){ ?>

//code to handle if customer just registered to your site

<?php } elseif($method == 'guest') {?>

//code to handle if customer is guest

<?php } else { ?>

//code to handle for logged in customer

<?php } ?>

$_customerId = Mage::getSingleton('customer/session')->getCustomerId();

$lastOrderId = Mage::getSingleton('checkout/session')->getLastOrderId();

$order = Mage::getSingleton('sales/order');

$order->load($lastOrderId);

$_totalData =$order->getData();

$_sub = $_totalData['subtotal'];

$_orderEmail = $_totalData['customer_email'];

$_orderNumber = $_totalData['increment_id'];

$installer = $this;

$installer->startSetup();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$entityTypeId     = $setup->getEntityTypeId('customer');

$attributeSetId   = $setup->getDefaultAttributeSetId($entityTypeId);

$attributeGroupId = $setup->getDefaultAttributeGroupId($entityTypeId, $attributeSetId);

$setup->addAttribute('customer', 'your_attribute_code_here', array(

    'input'         => 'text', //or select or whatever you like

    'type'          => 'int', //or varchar or anything you want it

    'label'         => 'Attribute description goes here',

    'visible'       => 1,

    'required'      => 0, //mandatory? then 1

    'user_defined' => 1,

));

$setup->addAttributeToGroup(

 $entityTypeId,

 $attributeSetId,

 $attributeGroupId,

 'your_attribute_code_here',

 '100'

);

$oAttribute = Mage::getSingleton('eav/config')->getAttribute('customer', 'your_attribute_code_here');

$oAttribute->setData('used_in_forms', array('adminhtml_customer')); 

$oAttribute->save();

$setup->endSetup();

$coll = Mage::getModel('sales/order')->getCollection();

$fn = Mage::getModel('eav/entity_attribute')->loadByCode('1', 'firstname');

$ln = Mage::getModel('eav/entity_attribute')->loadByCode('1', 'lastname');

$coll->getSelect()

    ->join(array('ce1' => 'customer_entity_varchar'), 'ce1.entity_id=main_table.customer_id', array('firstname' => 'value'))

    ->where('ce1.attribute_id='.$fn->getAttributeId()) 

    ->join(array('ce2' => 'customer_entity_varchar'), 'ce2.entity_id=main_table.customer_id', array('lastname' => 'value'))

    ->where('ce2.attribute_id='.$ln->getAttributeId()) 

    ->columns(new Zend_Db_Expr("CONCAT(`ce1`.`value`, ' ',`ce2`.`value`) AS fullname"));

print_r($coll->getData());