One common task that people want to do is to show login screens/sign up screens/etc., before their main application shows up, which often includes a tab bar:
The problem here is a tab bar should be your root controller, so once you load it, it's very difficult to hide it and show screens above it. Turns out, the solution is to not load it until it's needed.
We got a request the other day on how to do this, so @MikeB created a recipe and sample app illustrating how to do it. you can find it here: http://docs.xamarin.com/ios/recipes/content_controls/other_ux/use_initial_screen_in_tab_controller#pageFiles