The iPhone revolutionized the automatic screen rotation with its motion sensor. The display adjusts between landscape and portrait mode promptly depending upon the position, the phone is held.
Prior to iOS 4, there was no way to lock this automatic rotation and sometime it could be annoying, in particular, when you read on the device while laying down. (The iPad, running on iOS 3, has a physical switch for this).
Do you know, in the new iOS 4, iPhone user can easily lock or unlock the automatic screen rotation just by double clicking the home button and slide the bar toward the left , you will find the icon for that function. A status of current setting will then be indicated on the top bar.
Double clicking the home button also allows you quickly to control audio playback, should you have audio app plays in the background. I also find the multitasking bar useful to either switch to different apps or easily launch a recent app, especially the app icon is buried in a hard to find folder.
You can find other iPhone shortcuts HERE!