I know this is gonna sound wishy-washy and vague, but the hard thing to swallow in the applications of artistic photography is that it is an amalgamation of multiple disciplines.

When someone paints in oils, no one can ask, "well, was it REALLY oils, or a combo of something else?" For something like that it's just a matter of subject (Landscape, Still-Life, etc.), and they used oils for the medium.

With photography... It just takes off, really... It doesn't stop with the camera - talking even film use, there's over and under development, there's different types of contrasty papers to give you entirely different results - and even THOSE can be over and under developed.

Now we have the "digital darkroom" which blurs the line even more - SO much capabilities within a few clicks of the mouse. Some folks just do a Levels/Curves/Hue-Sat adjustment and call it a day (if they even do THAT). Some will do an extra (insert adjustment here) to the sky to bring out the clouds, maybe one for the shadow areas - is this correction, or manipulation...? Kinda like dodging and burning - many considered it manipulation, but it was a standard procedure for most printers using a difficult negative.

When you start adding/removing elements is when it gets muddy. A little here, a little there and some people (myself included) just take that as correction and blemish removal - on the flip side, others consider that blatant manipulation and scream that they feel deceived.

When it gets to be enough, like TomDart says above, "I know it when I see it.." And for the most part (yes, there are always exceptions), those that do extensive (meaning adding/removing/swapping elements) work to their images will put it in the Manipulation gallery (I just give up and put it in the 2D gallery - no one can really argue it when it goes in there).
-Lew ;-)