Everyone is going to have a different threshold for this sort of thing. In my opinion, adjustments to exposure, colour, contrast, sharpness, tone, dust spots, etc. are always acceptable, whether it be in journalism or art. Those adjustments are really not any different than picking a certain type of film that is, for example, more saturated.
Other alterations (cloning, removing things, changing things, etc.) fall into an ethical grey area for journalism. As with any ethical question, the standard will change depending on who you ask. Some will say that no alterations should be done to maintain journalistic integrity. Others don't mind a few things as long as the context of the image remains intact.
And, as far as art is concerned - have at it. Go wild. If it is overdone, some people will call bull on it and some people will lap it up like it is going out of fashion.
Just my opinion though...