In iOS 5.0, when you double-tap on the home button while your iPhone is locked, you get the above lock screen with a camera icon on the right end. You can then tap on the camera icon to activate the camera.
With iOS 5.1, Apple removes the double-tap gesture. Instead the camera icon is permanently shown in the lock screen. Tapping on the icon however will not activate the camera. You need to tap on the icon and swipe up to launch the camera app.
This might not look as easy as a single tap, but it is intuitive and is actually faster than double-tapping to launch the camera.