Its been some time when apple rolled out the feature of camera shortcut which gets displayed on iphone lock screen when you double click the home button with your iphone already locked, I never used this shortcut to access the camera as it was never quick way to open the camera.
In the recent update of ios 5.1 apple has made another update and fix the camera shortcut which now appears always on the lock screen, and in order to launch the camera new slide-up gesture is required to activate. (But for iPhone and iPod Touch 4th generation only and upwards). If you happen to use iPhone 3g or 3gs you will be required to double click the home button to show the camera on the lock screen.
What If I Have iOS 5.0.1 or Earlier?
The new feature only works on 4th generation iphone and ipod with ios 5.1 now if you want the same feature on your older iphone, ipod which is running older version of firmware but you iphone or ipod must be already jailbroken. Camera Gabber is a new tweak on cydia. It does not install any new icon on the home screen. It just add the same camera shortcut on the lock screen with the same slide to unlock text quite shortened and camera icon on the right with the same gesture new slide up to launch the camera as shown in the image below.
Please note that this tweak only works on the devices which are running ios 5.0. This tweak comes for free under ModMyi repo. If you really want to see how this tweak works, you can watch the video below.