(via kb.mediatemple.net)
To test your website before DNS propagation has completed you can edit the HOSTS file on your local computer. Your computer will use the entries in your HOSTS file before it looks up the current information in DNS. This walkthrough will explain how to edit the HOSTS file on Windows 98/Me/2000/XP/2003/Vista computers as well as on Apple Mac OS 9 and OS X machines.