Very cool - Scott discusses a previously overlooked new feature in ASP.NET 2.0 (well, actually, Eric noticed it first).
Basically, if you place a file with this name in the root of a web application directory, ASP.NET 2.0 will shut-down the application, unload the application domain from the server, and stop processing any new incoming requests for that application.
ASP.NET will also then respond to all requests for dynamic pages in the application by sending back the content of the app_offline.htm file (for example: you might want to have a "site under construction" or "down for maintenance" message).
Very cool way to throttle a single application to take it out of service. I can assure you our web team is going to love this... especially if we can make sure installs don't require an IIS reset.