Here's a nice collection of articles on digital photo manipulation and the ethics of doing it.
Most interesting to me were:
1. A link to "Lying with Pixels" on real-time video manipulation, by Ivan Amato, well-known science writer.
2. A link to "Photo Fakery," on identifying false images, by ITT Industries space scientist Robert D. Fiete.
3. A link to "Kate Doesn't Like Photoshop," by Jeff Schew, at Photoshop.Com , which gives examples of photo alteration in newspapers and magazines and links to various articles discussing the ethics of this practice.
The first link is to the National Press Photographer Association's Digital Manipulation Code of Ethics; the association's general code of ethics is here: