On Till Klampaeckel's blog this recent post shows you how to (quick and easy) set up multiple Magento stores on different domains with the same codebase.

Multiple stores is probably the killer feature of the Magento Commerce store. It enables the needy to manage multiple stores through a single interface. Your very own mall in a box. It's also a management/deployment nightmaredream come true. A single piece of software powering multiple websites.

He sets up his directory structure and shows how to alias certain directories to ones in the local document root so that the application will find things correctly. Drop in a custom index.php "bootstrap" file to run the application and you should be all set.