Very interesting slide show from Slate.com. Some architectural photographs (such as the Nanpu Bridge in Shanghai, or the Flatiron building viewed from Madison Square Park) are going to be similar in tone and reference. Photographers also photograph similar subjects, whether consciously or not. The book The Ongoing Moment by Geoff Dyer looks at the history of photography from this vantage point: Examining, for instance, photographs of blind street musicians through time, from the 19th century forward.
At what point does a work of art stop being a reference and become plagiarism? If someone were to take pictures of cranes or graffiti using similar techniques to mine, I'd be all for it (these subjects lend themselves to these techniques so readily I'm surprised no one else is doing it). Whether I'm referencing or plagiarizing graffiti is a more difficult question, and is one of the reasons I've been taking more pictures of cranes recently. (Also, cranes don't send me snotty emails asking if I'm going to cut them in on the profits.)
ETA: Here is a link to a YouTube video (4:31) via chr0me_kitten that I think touches on the same subject.
A further edit: From apostle_of_eris comes this link, The Ecstasy of Influence by Jonathan Lethem