Apple documentation talks about the possibility that more than one user can use the same device so if you use game center you should authenticate the player every time the game becomes active. Think of two children who play your game on their mother's iPhone (one at a time). So if your game uses Game Center you have to force a game center logout & login every time your game becomes active. You also have to persist the game state for each user separately and switch states depending on which user logged in. I'm thinking of a game with game center leaderboards and achievements -- Julie wouldn't want to make an achievement and have it credited to her brother Josh.
I have lots of games on my devices and I don't think I've ever seen one that does this. But maybe I didn't notice. Do you do some kind of per-user switch like this? Or do you assume that only one person will be playing your game?