Yeah, this one might be at the top of the list for awhile if you know what I mean...
The short answer for me is that if you can do it in a darkroom, it's OK to do it in Photoshop. Other stuff too depending on circumstances. That also goes for some things you would do at time of exposure such as multiple exposures.
Personally, I draw the line at - for example - adding a full moon to a landscape shot. I did it once with film just to see how bad it really would look and was not disappointed! I don't do that with any form of photography (I shoot both film and digital). I also might clone out things like dust and lens flare or the blue wristband that was on my right wrist in my new avatar shot (long boring story). The recent LA Times story about the faked image from Iraq is really bad. My problem with that shot is that it was presented as the truth but obviously wasn't.
Some people don't realize that unsharp masking was originally done in the darkroom long before Photoshop existed. I don't have enough darkroom training to tell you exactly how it was done, but it's a similar result with a lot less work. That's probably true of most things in Photoshop. I have no problem with RAW and deciding the color temperature, etc of the image later. Some people may, but whenever you shoot color neg film and take it to a lab, they make all kinds of "corrections" - including filtering for color temperature - in printing.