Magento – Disable checkout button in cart

The observer receives as a parameter the order (or orders for multishipping) and the quote. Here is how you can get them

public function galileo_booking_request(Varien_Event_Observer $observer) {
    $order = $observer->getEvent()->getOrder(); //I don't know why this returns null for you. It shouldn't
    //or for multishipping
    //$order = $observer->getEvent()->getOrders(); //you should get an array of orders
    //for the quote
    $quote = $observer->getEvent()->getQuote();
}

If it doesn't work for you for the order, try with the quote. After getting the quote object your can loop through the items.

foreach ($quote->getAllItems() as $item) {
    $product = $item->getProduct();//if you need it
    //your magic here.
}

Just hookup your observer to the sales_order_place_before

app/code/local/my/module/etc/config.xml

<frontend>
    <events>
        <sales_order_place_before>
            <observers>
                <securityChecks>
                    <class>my_module/observer</class>
                    <method>placeOrder</method>
                </securityChecks>
            </observers>
        </sales_order_place_before>
    </events>
</frontend>

Perform your checks and raise an exception. Magento will return an error in json format to your checkout page and Magento's checkout will redirect you back to your cart; the order will not be processed and the cart (quote) will not be emptied.

app/code/local/my/Module/Model/Observer.php

class My_Module_Model_Observer
{
    public function placeOrder(Varien_Event_Observer $observer)
    {
        $order = $observer->getEvent()->getOrder();

        //perform checks

        if(!$passed)
        {
            Mage::throwException("An error ocurred");
        }

    }

}