Magento Checkout - fill address forms from Quote address (if possible) for guest customers

Mage_Checkout_Block_Onepage_Billing.php

/**
 * Mage_Checkout_Block_Onepage_Billing::getAddress() should be something like this
 * 
 * Similar for Mage_Checkout_Block_Onepage_Shipping
 */

public function getAddress()
{
    if (is_null($this->_address)) {
        $this->_address = $this->getQuote()->getBillingAddress(); //always take address from quote
        if ($this->isCustomerLoggedIn()) {
            if(!$this->_address->getFirstname()) {
                $this->_address->setFirstname($this->getQuote()->getCustomer()->getFirstname());
            }
            if(!$this->_address->getLastname()) {
                $this->_address->setLastname($this->getQuote()->getCustomer()->getLastname());
            }
        }
    }

    return $this->_address;
}