Asp.Net 2.0 comes with this cool feature, just place a file named app_offline.htm in the root folder of your web application, this will shut-down the application and also unload the application domain from the server, not only this it will stop processing any respond to all requests for dynamic pages and send back the content of app_offline.htm file.
So in the app_offline.htm you can put "Under Construction", "Will come back in 5 min" etc.
When you remove app_offline.htm file again the next request will be processed normal, it will load the app and start serving as you would have expected.
Asp.net team deserves a big treat to make life so simple....