I've played with this. You need to implement shouldRotateBlahBlahBlah: (sorry, no docs on this computer to get the proper name) in your view controller. All you need to do is add the method and return YES. One thing I found (and it took me a while to work this out) is that if you have a toolbar, you need to implement this in all view controllers run by that toolbar. Until I did that, nothing would rotate at all.
BTW, there's a WhichWayIsUp sample which shows a simple rotating view.