Kevin beat me to finding this new feature that Scott Guthrie just posted on, but it is very cool. If you place a file called App_Offline.htm in the root of your web application, it will shut down all incoming requests into the web server and not return any dynamic pages. This is a great feature when you want to pull your server down for maintenance.
Read the complete post at http://www.dotnettipoftheday.com/blog.aspx?id=45