How in your opininon should and iPhone app interface behave when one rotates the device?
Should it adapt: should some elements go from upper part of display (portrait) to the side (landscape)?
Should it stay locked in app's native orientation?
Is there any common practise
Your question doesn't have a single definitive answer and is entirely dependent upon what you want to accomplish. Sometimes when the orientation changes, you want to respond with manually moving elements to different positions with the view. Other times, you can depend fully on the auto resizing mask of the views. This is actually preferred if you can get away with it. Sometimes you can't. Of course, there are also times when you wouldn't want to allow orientation changes at all which is something also allowed by the SDK.
I suggest you try some things and see what kinds of problems you run into and then post specific questions that the community could help you with.
Best regards.