<?php $_items = $this->getItemsCollection() ?>
    <?php $i=0;foreach ($_items as $_item):?>
        <?php if ($_item->getParentItem()) continue; else $i++;?>
        <tbody class="<?php echo $i%2?'even':'odd' ?>">
            <?php echo $this->getItemHtml($_item) ?>
            <?php echo $this->getItemExtraInfoHtml($_item) ?>
        </tbody>
    <?php endforeach; ?>

with

<?php $_items = $this->getItemsCollection() ?>
    <?php $_sortedItems = array(); ?>
    <?php foreach ($_items as $_item) : ?>
        <?php $_sortedItems[$_item->getSku()] = $_item;?>
    <?php endforeach;?>
    <?php ksort($_sortedItems); // This is where the sorting by SKU takes place ?>
    <?php $i=0;foreach ($_sortedItems as $_item):?>
        <?php if ($_item->getParentItem()) continue; else $i++;?>
        <tbody class="<?php echo $i%2?'even':'odd' ?>">
            <?php echo $this->getItemHtml($_item) ?>
            <?php echo $this->getItemExtraInfoHtml($_item) ?>
        </tbody>
    <?php endforeach; ?>

Done. If you open the detail page of an order with 2+ items, they should be sorted by SKU.