I don't know about identifying things after the fact, but an idea I've tossed around for a few years now is a forensic digital camera.
Basically, the hardware will sign/watermark all the photos it takes with some sort of digital signature unique to the camera. The private key would be buried in silicon in such a way as to destroy it if attempts are made to discover it.
I'm not a security/encryption expert by any means, so I don't know how feasible this is (or does it already exist?) but sounds plausible to me.