iOS 5 gives you access to the camera app directly from the lockscreen, which makes taking a photo much quicker than having to first unlock your iDevice and then flicking to find your camera icon.
When on your lockscreen, press the home button twice. In previous versions of iOS this would give you access to the audio playback controls. It still does, but new in iOS 5 is an additional button labelled 'Take Picture'. The button is to the right of the slider control that unlocks the iDevice, and a double-tap will open the camera app.
Also worth remembering is that you can now use the 'Volume Up' button on the side of your iDevice as a shutter release.