Creating your own container view controllers, i.e. view controllers that can contain other view controllers, used to be a pain but now iOS 5 makes it a lot easier.
Especially for iPad apps that is great, because all of that screen space means you probably want to divide it up amongst more than one view controller.
Today I was messing around with these new container APIs and built my own tab bar controller. It works just like a regular UITabBarController, except the tabs look quite different and the pages slide in and out of the screen with a nice animation.
If you want to learn more about how to do this, then check out the source code or get the iOS 5 by Tutorials ebook. It has a full chapter on these new view controller containment APIs, plus twenty or so other chapters on more iOS 5 goodness!