The NetTuts.com site has posted a tutorial (the third and last in their series) showing how to override controller handling in Magento. In the previous posts they showed how to override functionality related to the models and blocks (frontend layout elements).

In Magento, the controller is responsible for handling incoming requests, and it's a backbone of the Magento routing implementation. [...] As I said in the previous tutorial, it's never recommended to change core files directly, as it makes upgrading Magento really difficult. To avoid this, we should follow the standard way of making desired changes to core files: we should either use event observers or override core files with our custom module files. We'll discuss the overriding feature today.

You'll need to be familiar with custom module creation to be able to follow along (see here if not) but other than that they provide everything you'll need. They start by creating the files and folders needed for the custom module including:

• Module XML definition (Envato_All.xml)
• Module XML configuration
• the Envato_Catalog_ProductController controller file (PHP)

The controller extends the pre-existing Product controller but the configuration definitions tell it ti use the "Envato" version instead.