When I played the game I had been building for about 10 minutes or so, and when I went to the menu and came back later everything I had done was gone. This happens every time I try to go onto one of my (survival or creative) worlds. In survival mode if you craft something and place it then leave and return or even just placing something and exiting, you lose whatever you placed, it is gone from your inventory. It isn't your whole inventory that gets cleared it's just whatever item you placed and however many you placed, vanish when you return to the game. This bug renders the game almost unplayable as anything you create is deleted.
(On a side note my iPad is completely updated and the iOS version is 6.1)