Magento Onepage Checkout, Quick But Not Dirty
The first place to look is of course the Mage_Checkout_OnepageController, this is pretty much the bread and butter of many nice little tricks.
The index action together with the corresponding block and template file are indeed very important here. Indeed, our first treat today lies in the block file (Checkout/Block/Onepage.php):
[codesyntax lang="php" lines="fancy"]
$stepCodes = array('billing', 'shipping', 'shipping_method', 'payment', 'review');