Just be careful when you do this. It's really easy to get lost and frustrated in an app that doesn't have a consistent tab bar at the bottom. The problem is that you can click a few things, forget the context of what you were doing, and not have an easy way give you some context (the selected tab) or start over.

That being said, Apple does use that structure in the iTunes app when you're playing a song. However, from that view you cannot push anymore view controllers: the back button will always take you back to a screen showing the tab bar.