On iPhone and iPod Touch platform, View class, i.e. UIView, is a super class of the UIWindow. Because of the fact, UI design is also affected.
For Mac, a Window contains its content view, and you put other views on it. However, for the iPhone and iPod Touch, a view controller loads a view. Therefore, in a xib or nib file for the view, you put other views on the view loaded by the view controller. Well, let’s take a look at it.
So, it is easy way to understand that it is like to treat the view loaded from a nib/xib file like a window for Mac.
What is interesting about this is a portable code and UI design for Mac and iPhone/iPod touch at the same time. By making the programming model and UI design identical, it will be possible to write as portable code as possible.
I found good explanation on NSViewControllerView, this posts are good.