The recently released iOS 4.3 brought with it a number of welcome improvements. Perhaps one of the handiest and most requested was the ability to set the side switch as a rotation lock. iOS 4.2 had assigned this switch a mute function to silence app sounds. The rotation lock became a soft setting in 4.2, found with the fast app switcher. So rotation lock was available but not convenient. Version 4.3 of the iOS fixes this.
You can choose what you want the side switch to do, either mute or rotation lock. Go to
Settings > General and look for “Use Side Switch to”