Views 2 - Embed a View inside Empty Text of another View
In D7, Views 3 you can enable "NO RESULTS BEHAVIOR" which is under the "Advanced" tab.  Here you can add "Global: View area" (or just "hello world" as a text area for testing) which will allow you to display a view when there isn't any results.

If you want this is code, you can always export the View after it is configured and see how it was done.