If you want to instantly open your most frequent use app on your iPhone then there is a new application called AnyLockApp that allows you quick access to your favourite app straight from your iPhone’s lockscreen.
This new jailbreak tweak has released on Cydia and it is developed by Nick Frey. After installing this tweak on your iPhone, tap on Settings app from the homescreen of your iPhone to set your desired app. If you double tap from the lock screen, a camera button shows up and is available to use. Tapping on the camera button brings you right to the camera for quick and easy shooting.
You can install this tweak on your jailbroken iPhone by launching Cydia on your iPhone then search for AnyLockApp then click on “install” and then “confirm”. If you set passcode on your device then first you have to enter the passcode to open the app. The tweak is compatible on iOS 5 firmware running devices only.
You can download this app from here