The instant you do more than noise reduction, color correction (including tint, hue, contrast, color temperature, levels, etc), exposure correction (brightness, exposure, etc), correction of scratches/dust, or cropping. I might be convinced that cloning out minor things like power lines is acceptable too.
But I come from a background of shooting slides, so my tendency will probably always be that a photograph is as "straight out of the camera" as possible. It's unfortunate that digital, unlike slide film, requires so much additional correction after the shot. In terms of simply creating the photograph, slides are almost infinitely superior to digital IMO. Unfortunately storage of slides and sharing of same is a royal pain, and digital is wonderful for both.
ZV