I think this question is not nearly as simple as some people might believe, and in this post I will attempt to address some of the point that, I believe, are relevant. I don't think I can provide all possible answers, though. So what you find below is the current status of my thinking about this - not more. As always, I'd be happy to learn what I am missing.
First, what is plagiarism? dictionary.com defines plagiarism as "the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work." (I'm sure other dictionaries will yield a very similar result [and note, that's not plagiarism]). Note that for photography, we will have to replace "language" with "photography" - and this is partly why things will get a bit tricky.
But let's start with a very simple - and, I think, very obvious - example. Shown above you can see two pairs of images. The left column shows photos by Peter Bialobrzeski. The right column contains photos by Horst and Daniel Zielske, a father-and-son team. Peter Bialobrzeski's photos are part of his series "Neon Tigers"; the book, published in 2004, won various international awards, and the work was widely exhibited. Horst and Daniel Zielske's work just went on show at Hamburg's Museum für Kunst und Gewerbe (MKG). In a letter, Peter Bialobrzeski's representing galleries and publishers write that "Horst und Daniel Zielske certainly knew of the work and of the individual photographs, as before their first trip to Shanghai they had called Peter Bialobrzeski and asked for advice between colleagues, which he had provided as a colleague. When Peter Bialobrzeski became aware of the MKG circulating press pictures very similar to some of his own work, he asked the MKG to remove the photographs in question from the planned exhibition. Concerning details, colors and lighting, the Zielskes' 'Nanpu Bridge, Shanghai 2002' and 'Xuhui II, Shanghai 2005' clearly [...] bear resemblance to a work which is well-established on the international art market as an original statement, both aesthetically and in content, and especially since the 2004 exhibition in Hamburg and the corresponding book publication. Father and son Zielske nonetheless beg to differ. Their creed is stated in a press release [...] as follows: 'One possible explanation: There are innumerable moments - but only a few good ones. To see that one moment coming which counts, that is the secret of Horst und Daniel Zielske.'" (I am quoting this from an email, I yet have to find a link to a website that gives the letter in full - if someone wants to send me that, I'd appreciate it)
If you believe that the Tooth Fairy, Santa Claus, and the Easter Bunny really exist, you might find the response from the MKG's press release credible. For the rest of us - and for those who love to apply Occam's Razor - it looks like we are dealing with a fairly obvious case of plagiarism here. We do not know the intentions of Horst and Daniel Zielske when they took those photos, but the similarities between their photos and those by Peter Bialobrzeski are way too close. Even if Horst and Daniel Zielske had never seen Peter Bialobrzeski's photos I don't think theirs should go on display.
But the question remains where similarities end and plagiarism begins, and this is not always a very simple question. Again, let's start simple. Take a look at the two very similar photos shown in the top row and compare them with this photo by Edward Burtynsky. It looks like that's the same highway spiral and bridge, but that's where the similarities end. Clearly, that's not plagiarism.
And I think we might want to be a bit careful with the word plagiarism. Even though I realize that it has become very popular to throw around big words and accuse someone of this and that (stealing elections or files online are just obvious examples), proving plagiarism is actually a very tricky business, and given the severeness of the accusation, it does not hurt to be a bit careful. Also, if something is quite similar, that's not already plagiarism - especially if the methods are the same, but not the results.
For example, compare the work of Idris Khan and Jason Salavon. Both have used digital superimpositions of images to create new work, with different subject matters. The methods are similar, the results and intentions aren't.
Or take the iconic portraits done by Richard Avedon. If I was to take a portrait of someone, using an empty, white background, would that be plagiarism? Obviously not. While the method "invented" by Richard Avedon has been quite widely mimicked, this is not plagiarism, because, after all, where would this end? If you really could only take photos that are completely different from all exisiting photographs, that would limit what one could do quite immensely. What is more, just like any other art form, photography does evolve by mutation, and by that I mean that a photographer takes an existing technique or method and changes it to, ultimately, create something new.
Another problem is the following one: Imagine two photographers come up with the same idea. Without knowing that there is someone else doing the exact same thing, they go out and take photos. Given that digital technologies are constantly lowering the threshold to take high quality photographs and to show them to large audiences, I think this scenario will become increasingly common. It would be far-fetched to apply the "plagiarism" label here.
So what is plagiarism? If photographer A calls photographer B and then takes the exact same pictures (or pictures with minute differences), I think that looks like plagiarism. But one must be careful with many, maybe even most other cases.
And just as some sort of post-script, I think one has to be a bit careful about limiting oneself too much. If you see something that you think would make a great photo, then you want to take that photo - regardless of whether you think/know someone else has done it already. It might turn out quite different from that other photographer's work - unless, of course, you do the exact same thing and make it look like that someone else's work.
Update (18 Sept 06): Read an article about the Shanghai photos here. Note how according to some people if you complain about what looks like plagiarism you might just be "an offended prima donna." It's getting quite classy! Also see this brief write-up, where Peter Marshall disagrees with my assessment. And it is very important to take his point very seriously: If you take a photo of a landmark, is it even possible (or maybe more accurately asked: under what circumstances is it possible) to plagiarise somebody else's work? This question is quite relevant for the bridge, but less so for those apartment blocks (or maybe not?).
