I am experimenting with Interface Builder in Xcode 4.2 and iOS 5 on a old Mac.
In Xcode 3.2 there was always a MainWindow.xib file and it contained not only the File's Owner which was UIApplication IMHO, but also an App Delegate placeholder object.
I created a MainWindow.xib file using the Window xib template. The AppDelegate placeholder is missing, so I can't hook the window up to my actual app delegate object.
I suppose that I will need to add a placeholder object with it's class identity set to "AppDelegate" but couldn't figure out how.