Know this is an old post, but this is a very common problem for the Canon S2 and S3 IS series cameras, and there are likely others reading this with the same problem. But happily, it's easily fixed without even putting a screwdriver to the camera. A stuck shutter is a common failure mode for digital cameras. The symptoms of a stuck or "sticky" shutter are very similar to CCD image sensor failure. The camera may take black pictures (for shutter stuck closed), or the pictures may be very bright and overexposed, sometimes with lines, especially when taken outdoors (for shutter stuck open).
To confirm a stuck shutter, put the camera in any mode other than "Auto", and turn the flash OFF (you don't want to blind yourself for the next step). Next look down the lens and take a picture. You should see a tiny flicker in the center of the lens as the shutter opens and closes. If no movement is seen, then you likely have a stuck shutter. If so, please [http://camerarepair.blogspot.com/2009/05/simple-fix-for-stuck-shutter.html|see this link] for further info and simple troubleshooting steps to correct the problem.