Ive fixed a large number of iPhone 4's and this has happened only a few times to me also. I have yet to figure out the reason. I've tried replacement cameras, different screens, triple checking connections. It always seems to get stuck when you switch to the front camera. The downside to the preinstalled camera app is that if you switch to the front camera and the shutter gets stuck you cant seem to switch back. Go download camera+ or another camera app and unfortunately dont use the front camera...