<sales_quote_collect_totals_before>
<observers>
<hackathon_presentation>
<type>singleton</type>
<class>modulename/observer</class>
<method>salesQuoteAddressCollectTotalsBefore</method>
</hackathon_presentation>
</observers>
</sales_quote_collect_totals_before>

public function salesQuoteAddressCollectTotalsBefore($observer)
    {
        $quote = $observer->getQuote();
        $quote_items = $quote->getItemsCollection();
        foreach ($quote_items as $item) {
            $additionalOptions = array(
                array(
                    'code'  => 'my_code',
                    'label' => 'This text is displayed through additional options',
                    'value' => 'ID is ' . $item->getProductId() . ' and SKU is ' . $item->getSku()
                )
            );
            $item->addOption(
                array(
                     'code'  => 'additional_options',
                     'value' => serialize($additionalOptions),
                )
            );
        }
    }