The iOS HIG ( http://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/MobileHIG.pdf ) doesn't specifically say about the back button on the directly accessible screen, however, just because it happens to be possible to get back to the main screen via the main menu, that doesn't mean that's the best way to do it.

While in the app, returning from deeper screens using the back button should stop at the main screen otherwise you'd go back, back, back - oh! No back button! Context switch - Think - How do I get back to the top - I guess I'll have to use the main menu button.

So I think it *should* be there.