Your camera may be leaving a digital fingerprint. Researchers at Binghamtom University in New York have filed a patent for a technique which lets them extract unique identifying information from digital cameras based on pixel noise. Because of a whole mess of variables in the manufacturing process, each digital camera generates it own unique digital fingerprint. If the researchers are able to look at multiple images from a single camera they can apparently trace it back to the camera that took the picture. The researchers ran tests with 2,700 cameras and were able to match the photographs to the cameras 100% of the time.
The obvious use for this would be in law enforcement, where it could be used to identify bad guys such as child pornographers and forgers. The fingerprint can also be used to detect whether a photo has been tampered with. Since changing the photo with Photoshop would change the fingerprint of the photo, researchers could also tell if someone was trying to pass off a doctored photo as authentic.