For some reason my Mussel Wind application was running fine without a root view controller, but with the annoying "Applications are expected to have a root view controller at the end of application launch" message. I vaguely remember autorotation being somewhat tricky, but in the end it was working and all was good.
Until I got the request to add zoom to the large version of the web cam view (you get it when you tap the image). Put in the scroll view, set it to zoomable, added the tap gesture recognizer. All good. Except nor more autorotation. Put in all the debugging messages for rotation masks, and shouldRotate etc. All called, but only on creation, not when the device is rotated. Nada.
On a whim, decided to fix the missing root view controller message by adding the view controller manually. First, no joy because UIKit was sending a "viewControllers" message that wasn't being handled. This despite the fact that my class is a straight UIViewController subclass and rootViewController is documented as taking just that. Oh well, just implement that as returning an empty NSArray and presto: autorotation!