I've thought about this a lot too as I've been making the transition to digital photography and playing with compositing and different things.
My final conclusion is -- as long as you understand that this is no longer a photograph, and do not present it as a photograph but as another piece of art I have on problem with it.
The point where it becomes a problem is finding that line where it is so manipulated that it stops being photography and starts being another art form. Some lines are easy -- compositing like this is obviously over that line, and is art not photography. No problem, these were never presented as photographs.
The infamous photographer that painted in extra rockets [edit - sorry they weren't even rockets, on the investigation they found out the "rockets" were really flares] (they went back and caught a few more of his -- there was an explosion where the smoke was enhanced to cover more area, and even on the small image shown on a newspaper you could see the clone lines,
here we go) was obviously crossing a line. Here is the
rocket photo. No debate that this was fraud.
Then you get the question marks -- I believe National Geographic was caught a little while ago adding elephants into a picture to make it look like a larger herd. But on the other hand, there was a famous picture they did years ago of a lion ripping into a carcass -- when asked how they got that amazing shot the photographers admitted they couldn't find any lions hunting, so they had brought a carcass then lured the lions in with meat until they were doing what they wanted.
Is that any less manipulated?