How can I persist some data with a Magento Quote Item for reference on checkout?

I need to persist some data associated to a product and then process it on checkout. This is internal data that isn't visible to the user, however it is calculated when a user is adding an item to the cart.

I've tried extending the Cart Model's addProduct() call and using:

$product->setData('some_var', $my_d开发者_Python百科ata);

However, on the checkout, when I do:

$items = $this->getOnepage()->getQuote()->getAllItems();

foreach($items as $item)
{
   echo $item->getData('some_var');
}

That item doesn't persist.

I assume it's not actually storing the item but saving all of the product id's and regenerating the collection.

Any advice on this would be greatly appreciated.

---

It's easier to add new attribute for product and then add quote item definition to the config.xml

<global>
    <sales>
        <quote>
            <item>
                <product_attributes>
                    <your_attirubute_code />
                </product_attributes>
            </item>
        </quote>
    </sales>
</global>

Also this attribute must used in product listing. ("Used in Product Listing" = Yes)