Adding Attribute To Category List Product Collection

Only a core set of attributes are added to the product collection (for the list page) by default. This bit of XML will allow you to add additional attributes to that collection:

<config>
    <frontend>
        <product>
            <collection>
                <attributes>
                    <some_attribute/>
                </attributes>
            </collection>
        </product>
    </frontend>
</config>