By this measure, judges who put their name on opinions widely understood to have been written by their clerks are not committing plagiarism, while professors who put their names on their research assistants' papers are. Neither Shakespeare nor Eliot was at fault because each borrowed openly, and enhanced the original. The reader must be deceived, and must act differently as a result - buy one book rather than another, assign some grade or promotion. This element of "reliance" lies at the heart of the legal definition of "fraudulent copying," a term more precise than the common idea of "copyright infringement."
As noted earlier on IPBiz, plagiarism and "copyright infringement" are entirely DIFFERENT concepts. Copyright infringement is well-defined, and does NOT require intent, but does allow an "independent creation" as a defense.
USC discusses an "acceptable plagiarism" here in a fall 2006 graduate student handbook.