This is easy in 2.0, but there's no built-in way to do it in 1.1. It might be easiest to handle in global.asax (look at the application event handlers) or the base page, but this is not something I can do for you.
But using these methods means that you can't take all these files down at the same time. The app_offline method in 2.0 will do it nicely.
http://weblogs.asp.net/scottgu/archi...09/442332.aspx