I want to be able to update my site in peace, without the world being able to watch all of the changes live. Is there some way I can use a .htaccess file in the root directory that will send ALL VISITORS to a specified "under construction" page, while still allowing me (or, technically, my IP address), to view the site as normal? Is this possible?

First thought: Put up a quick N dirty "Under construction" page at www.domain.com/index.html.

While you work on the site at /index2.html then just cp over to index.html when your done.