How do I lock rotation with iOS 4 / iOS 5?
The iOS 4 firmware update includes a rotation lock feature, similar to the rotation lock switch found on the first generation iPad.
iPhone 4 ships with iOS 4 and also has this capability. To enable rotation lock and prevent the screen from automatically changing orientation when the iPhone is turned:
1. Double click the home button to bring up the multitasking bar.
2. Swipe right until the last set of icons is displayed in the multitasking bar (these will include play/pause and skip buttons for iPod or Pandora).
3. The furthest left icon looks like a circular arrow - touch this icon to enable or disable rotation lock on the iPhone.
4. A rotation lock icon will appear in the status bar next to the battery charge icon when rotation lock is enabled. This looks like a circular arrow around a padlock.
When rotation lock is enabled apps will not respond to the accelerometer sensor.
UPDATE: The toggle for rotation lock has been moved in iOS 7. Check here for instructions on how to lock rotation with iOS 7.