Anyway: In the footer of my magento shop we might have a summary of the shopping cart,
which i simply put in the footer.phtml template file:

<?php if (Mage::getModel('checkout/cart')->getItemsCount() > 0): ?>
  <div class="footer_cart_widget">
    <a href="/checkout/cart" class="cart">
      <?php echo $this->__('Warenkorb') ?>:<br />
      <span class="cart_meta">
        <?php echo Mage::getModel('checkout/cart')->getItemsCount() ?>
        <?php if (Mage::getModel('checkout/cart')->getItemsCount() == 1): ?>
          Produkt: 
        <?php else: ?>
          Produkte: 
        <?php endif; ?>
        <?php echo $this->helper('checkout')->formatPrice(Mage::getSingleton('checkout/cart')->getQuote()->getGrandTotal()); ?>
      </span>
    </a>
  </div>
<?php endif; ?>

This means we might need to disable the caching for the footer HTML block,
so that my cart summary will stay dynamically generated.

This is how you can do it:

copy

app/code/core/Mage/Page/Block/Html/Footer.php

to

app/code/local/Mage/Page/Block/Html/Footer.php

and just comment out the following lines:

//$this->addData(array(
//    'cache_lifetime'=> false,
//    'cache_tags'    => array(Mage_Core_Model_Store::CACHE_TAG, Mage_Cms_Model_Block::CACHE_TAG)
//));

you could also put the cart values in the getCacheKeyInfo() method, but as this would result in caching a footer for any state of the shopping cart from my understanding, i don’t think it’s necessarily a good idea.