Just create a page named App_Offline.htm, and put in the root of your web. When ASP.NET 2.0 sees this file it will shut down the worker process and redirect all traffic to said page. I blogged about this here.

Doh! Noob! Guess I need to start reading that .NET 2.0 book I bought 3 months ago.

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Jason Honingford - Web & Software Developer
www.PortVista.com

That's definitely a handy feature, and is great for taking care of downtime when you're transferring a large amount of files. 

However, it still doesn't solve the issue of what to do while you're initiating the installation process.  In order to run the installation file, you'd have to remove the App_Offline.htm file, which puts you back to square one (or maybe square two, since you at least got the file transfer out of the way). 

How can you implement an "under maintenance" page or message while you're actively running the installation.  Some of the previously mentioned suggestions will work, but I do agree with the posters that have said it needs to be a function of the application itself that one can easily and just click a check box to take the site down for non admin users while the site is being upgraded.  I've seen this in other applications; for example, x-cart allows you to lock down the application by setting it offline in the backend, and anyone that wants to view the website needs to enter a key that you specify.