This tweak also allows you to lock the orientation of your iPhone into landscape. Switcher Rotator works with SBSwitcher, SwitcherPlus, SwitcherMod, and more. You should note that on icons will be added to the homescreen of your iPhone and also no option will be available to configure this tweak. Switcher Rotator is not compatible with iPad.
You can download this tweak from Cydia under BigBoss repository. It costs $0.99 USD. You will need to have a jailbroken iPhone to install this tweak. If you don’t have jailbroken iPhone then you can follow the step by step instructions posted here to jailbreak your iPhone on iOS 4.2.1 with Redsn0w.