Steve:
I have been involved in photography professionally since 1959 (including 7 years as an instructor in the Air Force photo school) and 4 years as Air Force director of photography in Europe. I have my share of hours in a darkroom and I know manipulation has always been possible, but digital makes it much easier and less costly.
Many organizations such as the National Press Photographer's Association, National Geographic Magazine are struggling with this problem as misuse of this technology has embarrased these organizations and called into question their creditability. Moving piece of trash from a scene is a bit different than moving an entire pyramid like National Geo did (they took incredi ble heat for that one).
All good historical writing includes sources and references for the content in order that it may be independently verified if called into question. Some people and some groups are calling for something like that for digital images. It would be like a list of references and sources at the end of a book. It would require the photograpehr or photo editor to describe what was done to the photo. I am not sure how I feel about this, but some are calling for it.