app/design/frontend/default/your-theme/layout/sales.xml

<sales_email_order_items>
    <block type="sales/order_email_items" name="items" template="email/order/items.phtml">
        <action method="addItemRender"><type>default</type><block>sales/order_email_items_order_default</block><template>email/order/items/order/default.phtml</template></action>
        <action method="addItemRender"><type>grouped</type><block>sales/order_email_items_order_grouped</block><template>email/order/items/order/default.phtml</template></action>
        <block type="sales/order_totals" name="order_totals" template="sales/order/totals.phtml">
            ...
            ...
        </block>
    </block>
</sales_email_order_items>