It's pretty obvious when you are using your iPhone to take a picture. You have to hold the phone in front of your face to focus it correctly and, unless you have it silenced, the camera app makes that recognizable shutter sound when you snap a picture. If you want to be a bit more covert with your photo-taking, you should check out SneakyPix from Skidmore Apps.
SneakyPix is a camera app that hides the fact that you are taking a picture. It automatically takes photos or videos while you pretend to be talking on the phone. It even displays a fake calling screen so the casual onlooker will think you are on a call.
To use SneakyPix, you have to silence your phone and remember to turn off the flash in the camera app. If you forget