It's fun to read about all the literary luminaries coming to the defense of Ian McEwan, in response to the complaint that he in some illegitimate way used phrases from Lucilla Andrews's memoir, "No Time for Romance," in his novel "Atonement." Of course, it is true that art makes use of the world it finds, and fiction writers must be granted license to at least echo phrases and passages in other works, especially when writing satire (not the case with "Atonement"). In this case, as McEwan explained in his Guardian article defending himself, he had explicitly given credit to Andrews's book as a source, at the end of "Atonement."
I do believe that the self-appointed plagiarism police need to be resisted sometimes. There are those who regard any use of a phrase or two as plagiarism, no matter how generic. I recently received anonymous emails from someone on a relentless campaign to discredit a certain eminent local professor, but when I laboriously compared the professor's allegedly plagiarized passages with the sections by the previous writer, the echoes were fragmentary and insignificant.
However, let the literary writers not be too easy on themselves. Originality is essential and sometimes requires care and commitment. If I am writing a historical novel, I can borrow period detail from older works and still use entirely my own language. In his letter supporting McEwan, Thomas Pynchon cites a particular borrowed phrase: "Gentian violet! Come on. Who among us could have resisted that one?" The answer is that anyone could have, who thought it was important to do so. According to Sarah Lyall's Times piece, McEwan didn't say he knowingly borrowed Andrews's words; he said it was inadvertant.
When real plagiarism happens -- as it apparently did not in this case -- there should be no excuses. In the historical profession, none is accepted. It's the author's responsibility to ensure that his or her work is original. See the statement of the American Historical Association, which I believe should apply to everybody: journalists, scholars, and novelists.