Locking the rotation with iOS comes in handy on the iPhone, preventing the orientation of the screen from changing unexpectedly when reading, for example. Accessing the rotation lock button from the home screen is simple, just bring up the multitasking dock by pressing the home button twice. Scroll from left to right and there's the orientation icon.
Apple has limited this feature to locking the orientation in portrait mode only, and the multitasking dock won't appear at all in the landscape orientation. Developer BTPankow has put an end to all this, releasing SwitcherLand to add horizontal orientation lock to the iPhone. This tweak makes it possible to bring up the multitasking dock even when using an app in landscape mode.
From there, locking the orientation in landscape mode is achieved by following the same steps to lock orientation as are used from portrait mode. Of course, only apps that actually switch orientation depending on the position of the iPhone are affected. When landscape orientation is locked, opening an app like Photos will automatically default to the horizontal orientation.
SwitcherLand can be found in the BigBoss repo on Cydia for free. Just search for SwitcherLand or browse to Sections -> Tweaks -> SwitcherLand. The package has no additional settings to configure, it just adds landscape access to the app switcher and orientation lock control. Installation of SwitcherLand requires a jailbroken device running iOS 5 or later.