So, how do we lock the screen rotation in iOS 4.2? It's actually pretty simple, now that I have figured it out... Double click on the home button to reveal the multitasking menu then swipe to the right. You will notice an icon that has a circular arrow. Press once to lock the screen (the icon will change to show a small lock in the middle), then press again to unlock the screen (the lock icon will disappear). You see, that simple.