Yeah the lack of a definition  is because it is next to impossible to define one... Feel free to offer one up that is acceptable to the rank and file!!

I don't think photo manipulation is either "right" or "wrong". It only crosses the line if you claim an image to be unmanipulated when it is not. The only reason I shy away from it in my own photography is because the image always loses a lot of emotion (for me) and becomes, well, commercial. That's not to say that all altered images lack emotion--sometimes they have more. It's just how I feel about my own work. 

Geert offers up  his view that it is less acceptable when " manipulation becomes the actual main subject of the image" That has been one way we use to exclude some works.. Adding or removing something from an image is no worse than doing so when taking the shot as we ask someone to move this way or that, or step out of the shot.. 

With the digital darkroom, there is now more powerful ways than ever before, darkroom boffins have always manipulated images... Truth and reality are more my own guidelines, and for many it is up to them to define their own..

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt