I cannot think of a way to do this, for several reasons: HTTP is a "pull" protocol; a user has to request something from the Web server in order to get information back. In effect, what you want to do is push a pop-up window to all users announcing that the server is going down. Even if you could do this, you probably don't want to, since it might overwrite something else the user is doing on a different Web site.
If anyone else knows a way to force a message out to all connected web sessions, please let me know.
This was first published in January 2004