As to the forensic evidence: seriously, by traditional means the evidence was always there: it was impossible to hide. With digital that is truly no longer the case. In a composite image, saved as a jpeg, what finite evidence there might have been is confounded by the compression. It is truly impossible to tell (unless someone overdoes the cloning)
It's worth pointing out that, at least in the US, a photograph isn't proof of anything in a forensic case. It is the testimony of an individual that validates the photograph as forensic evidence. There is certainly enough incidents of faked photography in criminal cases that the technology used to produce the photograph is irrelevant. It's easier to move a weapon into someone's hand at the crime scene than it is to successfully clone it into someone's hand, much easier.
It's really a bogus issue, which is why the primary focus in forensic circle regarding digital technology is is on resolvable detail, not use of post-processing. I have a real interest in evidence/forensic photography, and have been reading about it for years. Digital changes nothing.
Jeff Spirer
Photos: http://www.spirer.com
One People: http://www.onepeople.com/
Surfaces and Marks: http://www.withoutgrass.com
