I think the step to switch your web archive to be the default app is not necessary.
If you deploy whatever.war and sun-web.xml states that it should use the root context "/" than this webapp will get served automatically whenever anyone requests a url like "
http://www.yourserver.com". This is because your app uses the root context and thus "your web server is now running" shouldn't be display anymore.
Of course your web.xml should contain a "<welcome-file-list>"-element to point to the entry point of your app.
This is exactly what I am doing on my virtual servers (I use four virtual servers).
If I query Glassfish using "asadmin get *.default-web-module" I get a list that contains only empty entries. But any request to any of my domains gets answered by the respective web app.
Maybe my blog post about virtual servers might help:
http://weblogs.java.net/blog/writtmeyer/archive/2008/02/virtual_servers.html
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